# 1911 Encyclopædia Britannica/United States, The/Geology

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II.—Geology
All the great systems of rock formations are represented in the United States, though close correlation with the systems of Europe is not always possible. The general geological column for the country is shown in the following table:—
| Eras of Time. Groups of Systems. | Periods of Time. Systems of Rocks. | Periods of Time. Systems of Rocks. |
| Cainozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Present. |
| Cainozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Pleistocene. |
| Cainozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Pliocene. |
| Cainozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Miocene. |
| Cainozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Oligocene. |
| Cainozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Eocene. |
| | Transition (Arapahoe and Denver formations). | Transition (Arapahoe and Denver formations). |
| Mesozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Upper Cretaceous. |
| Mesozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Widespread unconformity. |
| Mesozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Comanchean (Lower Cretaceous). |
| Mesozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Jurassic. |
| Mesozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Triassic. |
| Palaeozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Permian. |
| Palaeozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Coal Measures, or Pennsylvanian. |
| Palaeozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Widespread unconformity. |
| Palaeozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Subcarboniferous, or Mississippian. |
| Palaeozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Devonian. |
| Palaeozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Silurian. |
| Palaeozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Widespread unconformity. |
| Palaeozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Ordovician. |
| Palaeozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Cambrian. |
| Proterozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Great unconformity. |
| Proterozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Keweenawan. |
| Proterozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Widespread unconformity. |
| Proterozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Upper Huronian. |
| Proterozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Widespread unconformity. |
| Proterozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Middle Huronian. |
| Proterozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Widespread unconformity. |
| Proterozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Lower Huronian. |
| Archeozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Great unconformity. |
| Archeozoic | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Archean  $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ Great Granitoid Series (intrusive in the main, Laurentian). Great Schist Series (Mona, Kitchi, Keewatin, Qumnissee; Lower Huronian of some authors). |
Archeozoic (Archean) Group.—The oldest group of rocks, called the Archean, was formerly looked upon, at least in a tentative way, as the original crust of the earth or its downward extension, much altered by the processes of metamorphism. This view of its origin is now known not to be applicable to the Archean as a whole, since this system contains some metamorphosed sedimentary rocks. In other words, if there was such a thing as an original crust, which may be looked upon as an open question, the Archean, as now defined, does not appear to represent it. The meta-sedimentary rocks of the Archean include metamorphosed limestone, and schists which carry carbonaceous matter in the form of graphite. The marble and graphite, as well as some other indirect evidence of life less susceptible of brief statement, have been thought by many geologists sufficient to warrant the inference that life existed before the close of the era when the Archean rocks were formed. Hence the era of their formation is called the Archeozoic era.
Most of the Archean rocks fall into one or the other of two great series, a schistose series and a granitoid series, the latter being in large part intrusive in the former. The rocks of the granitoid series appear as great masses in the schist series, and in some places form great protruding bosses. They were formerly regarded as older than the schists and were designated on this account “primitive,” “fundamental,” &c. They have also been called Laurentian, a name which is still sometimes applied to them.
Nearly all known sorts of schist are represented in the schistose part of the system. Most of them are the metamorphic products of igneous rocks, among which extrusive rocks, many of them pyroelastic, predominate. Metamorphosed sedimentary rocks are widely distributed in the schistose series, but they are distinctly subordinate to the meta-igneous rocks, and they are so highly metamorphic that stratigraphic methods are not usually applicable to them. In some areas, indeed, it is difficult to say whether the schists are meta-sedimentary or meta-igneous. The likeness of the Archean of one part of the country to that of another is one of its striking features.
The Archean appears at the surface in many parts of the United States, and in still larger areas north of the national boundary. It appears in the cores of some of the western mountains, in some of the deep canyons of the west, as in the Grand Canyon of the Colorado in northern Arizona, and over considerable areas in northern Wisconsin and Minnesota, in New England and the piedmont plateau east of the Appalachian Mountains, and in a few other situations. Wherever it comes to the surface it comes up from beneath younger rocks which are, as a rule, less metamorphic. By means of deep borings it is known at many points where it does not appear at the surface, and is believed to be universal beneath younger systems.
Locally the Archean contains iron ore, as in the Vermilion district of northern Minnesota, and at some points in Ontario. The ore is mostly in the form of haematite.
Proterozoic (Algonkian) Systems.—The Proterozoic group of rocks (called also Algonkian) includes all formations younger than the Archean and older than the Palaeozoic rocks. The term Archean was formerly proposed to include these rocks, as well as those now called Archean, but the subdivision here recognized has come to be widely approved.
The Proterozoic formations have a wide distribution. They appear at the surface adjacent to most of the outcrops of the Archean, and in some other places. In many localities the two groups have not been separated. In some places this is because the regions where they occur have not been carefully studied since the subdivision into Archeozoic and Proterozoic was made, and in others because of the inherent difficulty of separation, as where the Proterozoic rocks are highly metamorphosed. On the whole, the Proterozoic rocks are predominantly sedimentary and subordinately igneous. Locally both the sedimentary and igneous parts of the group have been highly metamorphosed; but as a rule the alteration of the sedimentary portions has not gone so far that stratigraphic methods are inapplicable to them, though in some places detailed study is necessary to make out their structure.
The Proterozoic formations are unconformable on the Archean in most places where their relations are known. The unconformity between these groups is therefore widespread, probably more so than any later unconformity. Not only is it extensive in area, but the stratigraphic break is very great, as shown by (1) the excess of metamorphism of the lower group as compared with the upper, and (2) the amount of erosion suffered by the older group before the deposition of the younger. The first of these differences between the two systems is significant of the dynamic changes suffered by the Archean before the beginning of that part of the Proterozoic era represented by known formations. The extent of the unconformity is usually significant of the geographic changes of the interval unrecorded by known Proterozoic rocks.
The Proterozoic formations have been studied in detail in few great areas. One of these is about Lake Superior, where the formations have attracted attention on account of the abundant iron ore which they contain. Four major subdivisions or systems of the group have been recognized in this region, as shown in the preceding table. These systems are separated one from another by unconformities in most places, and the lower systems, as a rule, have suffered a greater degree of metamorphism than the upper ones, though this is not to be looked upon as a hard and fast rule. The commoner sorts of rock in the several Huronian systems are quartzite and slate (ranging from shale to schist); but limestone is not wanting, and igneous rocks, both intrusive and extrusive, some metamorphic and some not, abound. Iron ore occurs in the sedimentary part of the Huronian, especially in Minnesota, Michigan, Wisconsin and parts of Canada. The ore is chiefly haematite, and has been developed from antecedent ferruginous sedimentary deposits, through concentration and purification by ground water.
The lower part of the Keweenawan system consists of a great succession of lava flows, of prodigious thickness. This portion of the system is overlain by thick beds of sedimentary rock, mostly conglomerate and sandstone, derived from the igneous rocks beneath. A few geologists regard the sedimentary rocks here classed as Keweenawan as Palaeozoic; but they have yielded no fossils, and are unconformable beneath the Upper Cambrian, which is the oldest sedimentary formation of the region which bears fossils. The aggregate thickness of the Proterozoic systems in the Lake Superior region is several miles, as usually computed, but there are obvious difficulties in determining the thickness of such great systems, especially when they are much metamorphosed. The copper of the Lake Superior region is in the Keweenawan system, chiefly in its sedimentary and amygdaloidal parts.
The Proterozoic formations in other parts of the continent cannot be correlated in detail with those of the Lake Superior region. The number of systems is not everywhere the same, nor are they everywhere alike, and their definite correlation with one another is not ​possible now, and may never be. The Proterozoic formations have yielded a few fossils in several places, especially Montana and northern Arizona; but they are so imperfect, their numbers, whether of individuals or of species, are so small, and the localities where they occur so few, that they are of little service in correlation throughout the United States. The carbon-bearing shales, slates and schists, and the limestone, are indications that life was relatively abundant, even though but few fossils are preserved. Among the known fossils are vermes, crustacean and probably brachiopods and pteropods.
The character of the sediments of the Proterozoic is such as to show that mature weathering affected the older rocks before their material was worked over into the Proterozoic formations. This mature weathering, resulting in the relatively complete separation of the quartz from the kaolin, and both from the calcium carbonate and other basic materials, implies conditions of rock decay comparable to those of the present time.
In all but a few places where their relations are known, the Proterozoic rocks are unconformable beneath the Palaeozoic. Where conformity exists the separation is made on the basis of fossils, it having been agreed that the oldest rocks carrying the Olenellus fauna are to be regarded as the base of the Cambrian system.
The Palaeozoic and later formations are usually less altered, more accessible, and better known than the Proterozoic and Archeozoic, and will be taken up by systems.
Cambrian System.—The lower part of the Cambrian system, characterized by the Olenellus fauna, is restricted to the borders of the continent, where it rests on the older rocks unconformable in most places. The middle part of the system, characterized by the Paradoxides fauna, is somewhat more widespread, resting on the lower part conformably, but overlapping it, especially in the south and west. The upper part of the system, carrying the Dicellocephalus fauna, is very much more extensive; it is indeed one of the most widespread series of rocks on the continent. The lower, middle and upper parts of the system all contain marine fossils. This being the case, the distribution of the several divisions indicates that progressive submergence of the United States was in progress during the period, and that most of the country was covered by the sea before its close.
The system is composed chiefly of elastic rocks, and their composition and structure show that the water in which they were deposited was shallow. In the interior, the upper part of the system, the Potsdam sandstone, is generally arenaceous. It is well exposed in New York, Wisconsin, Missouri and elsewhere, about the outcrops of older rocks. The system is also exposed in many of the western mountains or about their borders, especially about those the cores of which are of Archean or Proterozoic rock.
The thickness of the system has been estimated at 10,000 to 12,000 ft. in eastern New York, and almost as much in the southern Appalachian Mountains (Georgia and Alabama); but its average thickness is much less. In Wisconsin, where the Upper Cambrian only is present, the thickness is about 1000 ft. The greater thickness in the east appears to be due in part to the fact that an extensive area of land, Appalachia, lay east of the site of the Appalachian Mountains throughout the Palaeozoic era, and quantities of sediment from it were accumulated where these mountains were to arise later. The greatness of the thickness, as it has been measured, is also due in part to the oblique position in which the beds of sediment were originally deposited.
The Cambrian formations have not been notably metamorphosed, except in a few regions where dynamic metamorphism has been effective. The system is without any notable amount of igneous rock. As in other parts of the world, the system here contains abundant fossils, among which trilobites, brachiopods and worms are the most abundant. The range of forms, however, is great.
Ordovician System.—The succeeding Ordovician (Lower Silurian) system of rocks is closely connected with the Cambrian, geographically, stratigraphically and faunally. Its distribution is much the same as that of the Upper Cambrian, with which it is conformable in many places. The Ordovician system contains much more limestone, and therefore much less elastic rock, than the Cambrian, pointing to clearer seas in which life abounded. The succession of beds in New York has become a sort of standard with which the system in other parts of the United States has been compared. The succession of formations in that state is as follows:—
| Ordovician | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Upper Ordovician  (or Cincinnatian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Richmond beds (in Ohio and Indiana). |
| Ordovician | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Upper Ordovician  (or Cincinnatian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Lorraine beds. |
| Ordovician | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Upper Ordovician  (or Cincinnatian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Utica shales. |
| Ordovician | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Middle Ordovician  (or Mohawkian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Trenton limestone. |
| Ordovician | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Middle Ordovician  (or Mohawkian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Black River limestone. |
| Ordovician | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Middle Ordovician  (or Mohawkian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Lowville limestone. |
| Ordovician | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Lower Ordovician  (or Canadian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | Chazy limestone. |
| Ordovician | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Lower Ordovician  (or Canadian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | Beekmantown limestone. ( = Calciferous). |
The classification in the right-hand column of this table is not applicable in detail to regions remote from New York.
There is in some places an unconformity between the Richmond beds (or their equivalent) and underlying formations, and this unconformity, together with certain palaeontological considerations, has raised the question whether the uppermost part of the system, as outlined above, should not be classed as Silurian (Upper Silurian). Over the interior the strata are nearly horizontal, but in the mountain regions of the east and west, as well as in the mountains of Arkansas ​and Oklahoma, they are tilted and folded, and locally much metamorphosed. The outcrops of the system appear for the most part in close association with the outcrops of the Cambrian system, but the system appears in a few places where the Cambrian does not, as in southern Ohio and central Tennessee. The thickness of the system varies from point to point, being greatest in the Appalachian Mountains, and much less in the interior.
The oil and gas of Ohio and eastern Indiana come from the middle portion of the Ordovician system. So also do the lead and zinc of south-western Wisconsin and the adjacent parts of Iowa and Illinois. The lead of south-eastern Missouri comes from about the same horizon.
The fossils of the Ordovician system show that life made great progress during the period, in numbers both of individuals and of species. The life, like that of the later Cambrian, was singularly cosmopolitan, being in contrast with the provincial character of the life of the earlier Cambrian and of the early (Upper) Silurian which followed. Beside the expansion of types which abounded in the Cambrian, vertebrate remains (fishes) are found in the Ordovician. So, also, are the first relics of insects. The departure of the Ordovician life from that of the Cambrian was perhaps most pronounced in the great development of the molluscs and crinoids (including cystoids), but corals were also abundant for the first time, and graptolites came into prominence.
Silurian System.—The Silurian system is much less widely distributed than the Ordovician. This and other corroborative facts imply a widespread emergence of land at the close of the Ordovician period. As a result of this emergence the stratigraphic break between the Ordovician and the Silurian is one of the greatest in the whole Palaeozoic group.
The classification of the system in New York is as follows:—
| Silurian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Cayugan  (Neo- or Upper Silurian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Manlius limestone. |
| Silurian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Cayugan  (Neo- or Upper Silurian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Rondout waterlime. |
| Silurian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Cayugan  (Neo- or Upper Silurian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Cobleskill limestone. |
| Silurian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Cayugan  (Neo- or Upper Silurian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Salina beds. |
| Silurian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Niagaran  (Meso- or Middle Silurian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Guelph dolomite. |
| Silurian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Niagaran  (Meso- or Middle Silurian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Lockport limestone. |
| Silurian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Niagaran  (Meso- or Middle Silurian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Rochester shale. |
| Silurian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Niagaran  (Meso- or Middle Silurian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Clinton beds. |
| Silurian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Oswegan  (Palaeo- or Lower Silurian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Medina sandstone. |
| Silurian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Oswegan  (Palaeo- or Lower Silurian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Oneida conglomerate. |
| Silurian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Oswegan  (Palaeo- or Lower Silurian) | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Shawangunk grit. |
The lower part of this system is chiefly elastic, and is known only in the eastern part of the continent. The middle portion contains much limestone, generally known as the Niagara limestone, and is much more widespread than the lower, being found very generally over the eastern interior, as far west as the Mississippi and in places somewhat beyond. The Niagara limestone contains the oldest known coral reefs of the continent. They occur in eastern Wisconsin and at other points farther east and south. It is over this limestone that the Niagara falls in the world-famous cataract. One member of the middle division of the system (Clinton beds) contains much iron ore, especially in the Appalachian Mountain region. The ore is extensively worked at some points, as at Birmingham, Alabama. The upper part of the system is more restricted than the middle, and includes the salt-bearing series of New York, Ohio and Pennsylvania, with its peculiar fauna. It is difficult to see how salt could have originated in this region except under conditions very different climatically from those of the present time.
In the interior the thickness of the system is less than 1000 ft. in many places, but in and near the Appalachian Mountains its thickness is much greater—more than five times as great if the maximum thicknesses of all formations be made the basis of calculation. In the Great Plains and farther west the Silurian has little known representation. Either this part of the continent was largely land at this time, or the Silurian formations here have been worn away or remain undifferentiated. Rocks of Silurian age, however, are known at some points in Arizona, Nevada and southern California.
Corals, echinoderms, brachiopods and all groups of molluscs abounded. Graptolites had declined notably as compared with the Ordovician, and the trilobites passed their climax before the end of the period. Certain other remarkable crustacean, however, had made their appearance, especially in connexion with the Salina series of the east.
There are numerous outliers of the Silurian north of the United States, even up to the Arctic regions. These outliers have a common fauna, which is closely related to that of the interior of the United States. They give some clue to the amount of erosion which the system has suffered, and also afford a clue to the route by which the animals whose fossils are found in the United States entered this country. Thus, the Niagara fauna of the interior of the United States has striking resemblances to the mid-Silurian faunas of Sweden and Great Britain. It seems probable, therefore, that marine animals found migratory conditions between these regions, probably by way of northern islands. The fauna of the Appalachian region is far less like that of Europe, and indicates but slight connexion with the fauna of the interior. Both the earlier and the later parts of the Silurian period seem to have been times when physical conditions were such as to favour the development of provincial faunas, while during the more widespread submergence of the middle Silurian the fauna was more cosmopolitan.
Devonian System.—The Devonian system appears in some parts of New England, throughout most of the Appalachian region, over much of the eastern interior from New York to the Missouri River, in Oklahoma, and perhaps in Texas. It is absent from the Great Plains, so far as now known, and is not generally present in the Rocky Mountains, though somewhat widespread between them and the western coast. As a whole, the system is more widespread than the Silurian, though not so widespread as the Ordovieian. As in the case of the Ordovician and the Silurian, the New York section has become a standard with which the system in other parts of the country is commonly compared. This section is as follows:—
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Upper  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Chautauquan-Chemung (including Catskill). | Chautauquan-Chemung (including Catskill). | Chautauquan-Chemung (including Catskill). |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Upper  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Senecan | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Portage beds. |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Upper  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Senecan | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Genesee shale. |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Upper  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Senecan | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Tully limestone. |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Middle  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Erian | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | Hamilton shale. |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Middle  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Erian | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | Marcellus shale. |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Middle  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Ulsterian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Onondaga (Corniferous limestone) |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Middle  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Ulsterian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Schoharie grit. |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Middle  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Ulsterian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Esopus grit. |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Lower  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Oriskanian | | Oriskany beds. |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Lower  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Helderbergian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Kingston beds. |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Lower  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Helderbergian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Becraft limestone. |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Lower  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Helderbergian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | New Scotland beds. |
| Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Lower  Devonian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Helderbergian | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | Coeymans limestone. |
The formations most widely recognized are the Helderberg limestone, the Onondaga limestone and the Hamilton shale.
The Catskill sandstone, found chiefly in the Catskill Mountain region of New York, is one of the distinctive formations of the system. It has some similarity to the Old Red Sandstone of Great Britain. In part, at least, it is equivalent in time of origin to the Chemung formation; but the latter is of marine origin, while the Catskill formation appears to be of terrestrial origin.
No other system of the United States brings out more clearly the value of palaeontology to palaeogeography. The faunas of the early Devonian seem to have entered what is now the interior of the United States from the mid-Atlantic coast. The Onondaga fauna which succeeded appears to have resulted from the commingling of the resident lower Devonian fauna with new emigrants from Europe by way of the Arctic regions. The Hamilton fauna which followed represents the admixture of the resident Onondaga fauna with new types which are thought to have come from South America, showing that faunal connexions for marine life had been made between the interior of the United States and the lands south of the Caribbean Sea, a connexion of which, before this time, there was no evidence. The late Devonian fauna of the interior represents the commingling of the Hamilton fauna of the eastern interior with new emigrants from the north-west, a union which was not effected until toward the close of the period.
Like the earlier Palaeozoic systems, the Devonian attains its greatest known thickness in the Appalachian Mountains, where sediments from the lands of pre-Cambrian rock to the east accumulated in quantity. Here elastic rocks predominate, while limestone is more abundant in the interior. If the maximum thicknesses of all Devonian formations be added together, the total for the system is as much as 15,000 ft.; but such a thickness is not found in any one place.
The Devonian system yields much oil and gas in western Pennsylvania, south-western New York, West Virginia and Ontario; and some of the Devonian beds in Tennessee yield phosphates of commercial value. The Hamilton formation yields much flagstone.
Among the more important features of the marine life of the period were (1) the great development of the molluscs, especially of cephalopods; (2) the abundance of large brachiopods; (3) the aberrant tendencies of the trilobites; (4) the profusion of corals; and (5) the abundance, size and peculiar forms of the fishes. The life of the land waters was also noteworthy, especially for the great deployment of what may be called the crustacean-ostracodermo-vertebrate group. The crustacea were represented by eurypterids, the ostracoderms by numerous strange, vertebrate-like forms (Cephalaspis, Cyathaspis, Trematopris, Bothriolepis, &c.), and the vertebrates by a great variety of fishes. The land life of the period is represented more fully among the fossils than that of any preceding period. Gymnosperms were the highest types of plants.
The Devonian system is not set off from the Mississippian by any marked break. On the other hand, the one system merges into the other, so that the plane of separation is often indistinct.
Mississippian System.—The Mississippian system was formerly regarded as a part of the Carboniferous, and was described under the name of Lower Carboniferous, or Subcarboniferous, without the rank of a system. This older classification, which has little support except that which is traditional, is still adhered to by many geologists; but the fact seems to be that the system is set off from the Pennsylvanian (Upper Carboniferous) more sharply than the Cambrian is from the Ordovician, the Silurian from the Devonian, or the Devonian from the Mississippian.
​The system is well developed in the Mississippi Basin, whence its name. Its formations are much more widespread than those of any other system since the Ordovician. They appear at the surface in great areas in the interior, in the south-west and about many of the western mountains. In many places in the west they rest on what appear to be Ordovician beds, but without unconformity. The explanation of the apparent conformity of the strata from the Cambrian to the Pennsylvanian in some parts of the west, with no fossils defining with certainty any horizon between the Ordovician and the Mississippian, is one of the open problems in the geology of the United States.
The subdivision of the system for various regions in the eastern part of the United States is as follows:—
| Mississippi River States. | Ohio. | Pennsylvania. | Maryland. |
| 4. Kaskaskia or Chester | 7. Maxville | | |
| 3. St Louis | 6. Logan | | 3. Mauch Chunk |
| 2. Osage or Augusta (including the Burlington, Keokuk and Warsaw) | 5. Black Hand | 2. Mauch Chunk | |
| 2. Osage or Augusta (including the Burlington, Keokuk and Warsaw) | 4. Cuyahoga | | 2. Greenbrier |
| 2. Osage or Augusta (including the Burlington, Keokuk and Warsaw) | 3. Sunbury | | |
| 2. Osage or Augusta (including the Burlington, Keokuk and Warsaw) | 2. Berea grit | | |
| 1. Kinderhook or Chouteau | 1. Bedford | 1. Pocono | 1. Pocono |
In the interior the Kinderhook series has a distribution similar to that of the Devonian; the Osage series is more widespread, pointing to progressive submergence; and the St Louis is still more extensive. This epoch, indeed, is the epoch of maximum submergence during the period, and the maximum since the Ordovician. Before its close the sea of the Great Basin which had persisted since the Devonian was connected with the shallow sea which covered much of the interior of the United States. The fourth series, the Kaskaskia or Chester, is more restricted, and points to the coming emergence of a large part of the United States. In the Mississippi Basin the larger part of the system is of limestone, though there is some elastic material in both its basal and its upper parts. In Ohio the system contains much elastic rock, and in Pennsylvania little else. The Mauch Chunk series (shale and sandstone) is now believed to be largely of terrestrial origin.
The system ranges in thickness from nearly 5000 ft. maximum in Pennsylvania to 1500 ft. in the vicinity of the Mississippi river. In West Virginia some 2000 ft. of limestone are assigned to this system. The zinc and lead of the Joplin district of Missouri are in the limestone of this system, and the corresponding limestone in some parts of Colorado, as at Leadville, is one of the horizons of rich ore.
The end of the period was marked by the widespread emergence of the continent, and parts of it were never again submerged, so far as is known. Certainly there is no younger marine formation of comparable extent in the continent. When deposition was renewed in the interior of the continent, the formations laid down were largely non-marine, and, over great areas, they rest upon the Mississippian unconformably.
From the conditions outlined it is readily inferred that the faunas of the system were cosmopolitan. All types of life to which shallow, clear sea-water was congenial appear to have abounded in the interior. It was perhaps at this time that the crinoids, as a class, reached their climax, and most forms of lime-carbonate-secreting life seem to have thriven. Where the seas were less clear, as in Ohio, the conditions are reflected in the character of the fossils. Marine fishes had made great progress before the close of the period. Amphibia appeared before its close, and plant life was abundant and varied, though the types were not greatly in advance of those of the Devonian. The time of such widespread submergence was hardly the time for the great development of land vegetation.
Pennsylvanian System.—The Pennsylvanian or Upper Carboniferous system overlies the Mississippian unconformably over a large part of the United States. In the eastern half of the country the system consists of shales and sandstones chiefly, but there is some limestone, and coal enough to be of great importance economically, though it makes but a small part of the system quantitatively. The larger part of the system in this part of the country is not of marine origin; yet the sea had access to parts of the interior more than once, as shown by the marine fossils in some of the beds. The dominantly terrestrial formations of the eastern half of the country are in contrast with the marine formations of the west. The line separating the two phases of the system is a little east of the 100th meridian. West of the Mississippi the Coal Measures are subdivided into two series, the Des Moines below and the Missouri above. In the eastern part of the country (Pennsylvania, Ohio, &c.) the system is divided into four principal parts:—
| Pennsylvanian. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | 4. Monongahela formation (or series)—Upper Productive Coal Measures. |
| Pennsylvanian. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | 3. Conemaugh formation (or series)—Lower Barren Coal Measures. |
| Pennsylvanian. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | 2. Allegheny formation (or series)—Lower Productive Coal Measures. |
| Pennsylvanian. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | 1. Pottsville formation (or series). |
The Pottsville formation is chiefly elastic, and corresponds roughly to the Millstone Grit of England. The Allegheny and Monongahela series contain most of the coal, though it is not wanting in the other subdivisions of the system. Productive coal beds are found in five principal fields. These are (1) the Anthracite field in eastern Pennsylvania, nearly 500 sq. m. in extent; (2) the Appalachian field, having an area of about 71,000 sq. m. (75% being productive), and extending from Pennsylvania to Alabama; (3) the northern interior field, covering an area of about 11,000 sq. m. in southern Michigan; (4) the eastern interior field in Indiana, Illinois and Kentucky, with an area of about 58,000 sq. m. (55% being productive); and (5) the western interior and south-western field, some 94,000 sq. m. in extent, reaching from Iowa on the north to Texas on the south. There is also a coalfield in Nova Scotia and New Brunswick, about 18,000 sq. m. in extent. Some of the well-known beds of coal are known to be continuous for several thousands of square miles.
Unlike the older systems of the Palaeozoic, the Pennsylvanian system has not its maximum thickness in the Appalachian Mountains, but in Arkansas, in a region which was probably adjacent to high lands at that time. These lands perhaps lay in the present position of the Ouachita Mountains.
The close of the Pennsylvanian period was marked by the beginning of profound changes, changes in geography and climate, and therefore changes in the amount and habitat of life, and in the sites of erosion and sedimentation. One of the great changes of this time was the beginning of the development of the Appalachian Mountain system. The site of these mountains had been, for the most part, an area of deposition throughout the Palaeozoic era, and the body of sediments which had gathered here at the western base of Appalachia, by the close of the Pennsylvanian period, was very great. At this time these sediments, together with some of Appalachia itself, began to be folded up into the Appalachian Mountains. These mountains have since been worn down, so that, in spite of their subsequent periods of growth, their height is not great.
The chief interest of the palaeontology of this system is in the plants, which were very like those of the Coal Measures of other parts of the earth and showed a high development of forms that are now degenerate. Among land animals the amphibia had great development at this time. So also had insects and some other forms of land life.
Permian Period.—The Permian system appears in smaller areas in the United States than any other Palaeozoic system. The “Upper Barren Coal Measures” of some parts of the east (Ohio, Pennsylvania, &c.) are now classed as Permian on the basis of their fossil plants. They represent but a part of the Permian period, and are commonly described under the name of the Dunkard series.
The system has much more considerable development west of the Mississippi than east of it, especially in Texas, Kansas, Nebraska and beyond. Some of the Permian beds of this region are marine, while others are of terrestrial origin. In this part of the country the Permian beds are largely red sandstone, often saliferous and gypsiferous. They are distinguished with difficulty from the succeeding Triassic, for the beds have very few fossils. The system has its maximum known thickness in Texas, where it is said to be 7000 ft. in maximum thickness. West of the Rocky Mountains the Permian has not been very generally separated from overlying and underlying formations, though it has been differentiated in a few places, as in south-western Colorado and in some parts of Arizona. Perhaps the most remarkable feature of the palaeontology of the system is its paucity of fossils, especially in those parts of the system, such as the Red Beds, which are of terrestrial origin.
In the United States no direct evidence has been found of the low temperature which brought about glaciation in many other parts of the earth during this period. Salt and gypsum deposits, and other features of the Permian beds, together with the fewness of fossils, indicate that the climate of the Permian was notably arid in many regions.
Triassic System.—This system has but limited representation in the eastern part of the United States, being known only east of the Appalachian Mountains in an area which was land throughout most of the Palaeozoic era, but which was deformed when the eastern mountains were developed at the close of the Palaeozoic. In the troughs formed in its surface during this time of deformation, sediments of great thickness accumulated during the Triassic period. These sediments are now mostly in the form of red sandstone and shale, with conglomerate, black shale and coal in some places. These rocks do not represent the whole of the period. They are often known as the Newark series, and seem to be chiefly, if not wholly, of terrestrial origin. The sedimentary rocks are affected by many dikes and sheets of igneous rock, some of the latter being extrusive and some intrusive. The strata are now tilted and much faulted, though but little folded. In the western plains and in the western mountains the Triassic is not clearly separated from the Permian in most places. So far as the system is differentiated, it is a part of the Red Beds of that region. The tendency of recent years has been to refer more and more of these beds to the Permian. The ​Triassic system is well developed on the Pacific coast, where its strata are of marine origin, and they extend inland to the Great Basin region.
The climate of the period, at least in its earlier part, seems to have been arid like that of the Permian, as indicated both by the paucity of fossils and by the character of the sediments. The salt and gypsum constitute a positive argument for aridity. The character of some of the conglomerate of the Newark series of the east, and the widespread redness of the beds, so far as it is original, also point to aridity.
As in other parts of the earth, the Triassic was the age of gymnosperms, which were represented by diverse types. Reptiles were the dominant form of animals, and land reptiles (dinosaurs) gained over their aquatic allies.
Jurassic System.—This system is not known with certainty in the eastern half of the United States, though there are some beds on the mid-Atlantic coast, along the inland border of the coastal plain, which have been thought by some, on the basis of their reptilian fossils, to be Jurassic. The lower and middle parts of the system are but doubtfully represented in the western interior. If present, they form a part of the Red Beds of that region. On the Pacific coast marine Jurassic beds reach in from the Pacific to about the same distance as the Triassic system. The Upper Jurassic formations are much more widely distributed. During the later part of the period the sea found entrance at some point north of the United States to a great area in the western part of the continent, developing a bay which extended far down into the United States from Canada. In this great bay formations of marine origin were laid down. At the same time marine sedimentation was continued on the Pacific coast, but the faunas of the west coast and the interior bay are notably unlike, the latter being more like that of the coast north of the United States. This is the reason for the belief that the bay which extended into the United States had its connexion with the sea north of the United States.
The Jurassic faunas of the United States were akin to those of other continents. The great development of reptiles and cephalopods was among the notable features. At the close of the period there were considerable deformations in the west. The first notable folding of the Sierras that has been definitely determined dates from this time, and many other mountains of the west were begun or rejuvenated. The close of the period, too, saw the exclusion of the sea from the Pacific coast east of the Sierras, and the disappearance, so far as the United States is concerned, of the great north-western bay of the late Jurassic. Before the close of the period, the aridity which had obtained during the Permian, and at least a part of the Triassic, seems to have disappeared.
Comanchean System.—This system was formerly classed as the lower part of the Cretaceous, but there are strong reasons for regarding it as a separate system. Its distribution is very different from that of the Upper Cretaceous, and there is a great and widespread unconformity between them. The faunas, too, are very unlike. The Comanchean formations are found (1) on the inland border of the coastal plain of the Atlantic (Potomac series) and Gulf coasts (Tuscaloosa series at the east and Comanchean at the west); (2) along the western margin of the Great Plains and in the adjacent mountains; and (3) along the Pacific coast west of the Sierras. In the first two of these positions, the formations show by their fossils that they are of terrestrial origin in some places, and partly of terrestrial and partly of marine origin in others. In the coastal plain the Comanchean beds are generally not cemented, but consist of gravel, sand and clay, occupying the nearly horizontal position in which they were originally deposited. Much plastic clay and sand are derived from them. In Texas, whence the name “Comanchean” comes, and where different parts of the system are of diverse origins, there is some limestone. This sort of rock increases in importance southward and has great development in Mexico. In the western interior there is difference of opinion as to whether certain beds rich in reptilian remains (the Morrison, Atlantosaurus, Como, &c.) should be regarded as Jurassic or Comanchean. On the western coast the term Shastan is sometimes applied to Lower Cretaceous. In the United States, marine Shastan beds are restricted to the area west of the Sierras, but they here have great thickness.
Widespread changes at the end of the period exposed the areas where deposition has been in progress during the period to erosion, and the (Upper) Cretaceous formations rest upon the Comanchean unconformably in most parts of the country. The Comanchean system contains the oldest known remains of netted-veined leaved plants, which mark a great advance in the vegetable world. Reptiles were numerous and of great size. They were the largest type of life, both on land and in the sea.
Cretaceous System.—This system is much more extensively developed in the United States than any other Mesozoic system. It is found (1) on the Atlantic coastal plain, where it laps up on the Comanchean, or over it to older formations beyond its inland margin; (2) on the coastal plain of the Gulf region in similar relations; (3) over the western plains; (4) in the western mountains; and (5) along the Pacific coast. Unlike the Comanchean, the larger part of the Cretaceous system is of marine origin. The distribution of the beds of marine origin shows that the sea crept up on the eastern and southern borders of the continent during the period, covered the western plains, and formed a great mediterranean sea between the eastern and western lands of the continent, connecting the Gulf of Mexico on the south and the Arctic Ocean on the north. This widespread submergence, followed by the deposition of marine sediments on the eroded surface of Comanchean and older rocks, is the physical reason for the separation of the system from the Comanchean. This reason is reinforced by palaeontological considerations.
Both on the Atlantic and over the western plains. the system is divided into four principal subdivisions:—
| Atlantic Coast. | Western Plains. |
| 4. Manasquan formation. | 4. Laramie. |
| 3. Rancocas formation. | 3. Montana: Fox Hills; Fort Pierre. |
| 2. Monmouth formation. | 2. Colorado: Niobrara; Benton. |
| 1. Matawan formation. | 1. Dakota. |
The most distinctive feature of the Cretaceous of the Atlantic coastal plain is its large content of green sand marl (glauconite). The formations are mostly incoherent, and have nearly their original position. In the eastern Gulf states there is more calcareous material, represented by limestone or chalk. In the Texan region and farther north the limestone becomes still more important. In the western plains, the first and last principal subdivisions of the system (Dakota and Laramie) are almost wholly non-marine. The Dakota formation is largely sandstone, which gives rise to “hogbacks” where it has been tilted, indurated and exposed to erosion along the eastern base of the Rocky Mountains. The Colorado series contains much limestone, some of which is in the form of chalk. This is par excellence the chalk formation of the United States. That the chalk was deposited in shallow, clear seas is indicated both by the character of the fossils other than foraminifera and by the relation of the chalk to the elastic portions of the series. The Montana series, most of which is marine, was deposited in water deeper than that of the Colorado epoch, though the series is less widespread than the preceding. The Laramie is the great coal-bearing series of the west, and corresponds in its general physical make-up and in its mode of origin to the Coal Measures of the east. The coal-bearing lands of the Laramie have been estimated at not less than 100,000 sq. m. On the Pacific coast the Cretaceous formations are sometimes grouped together under the name of Chico. The distribution of the Chico formations is similar to that of the Comanchean system in this region.
The Cretaceous system is thick. If maximum thicknesses of its several parts in different localities, as usually measured, are added together, the total would approach or reach 25,000 ft.; but the strata of any one region have scarcely more than half this thickness, and the average is much less.
The close of the period was marked by very profound changes which may be classed under three general headings: (1) the emergence of great areas which had been submerged until the closing stages of the period; (2) the beginning of the development of most of the great mountains of the west; (3) the inauguration of a protracted period of igneous activity, stimulated, no doubt, by the crustal and deeper-seated movements of the time. These great changes in the relation of land and water, and in topography, led to correspondingly great changes in life, and the combination marks the transition from the Mesozoic to the Cainozoic era.
Tertiary Systems.—The formations of the several Tertiary periods have many points of similarity, but in some respects they are sharply differentiated one from another. They consist, in most parts of the country, of unconsolidated sediments, consisting of gravel, sand, clay, &c., together with large quantities of tuff, volcanic agglomerate, &c. Some of the sedimentary formations are of marine, some of brackish water, and some of terrestrial origin. In the western part of the country there are, in addition, very extensive flows of lava covering in the aggregate some 200,000 sq. m. Terrestrial sedimentation was, indeed, a great feature of the Tertiary. This was the result of several conditions, among them the recent development, through warping and faulting and volcanic extrusion, of high lands with more or less considerable slopes. From these high lands sediments were borne down to lodge on the low lands adjacent. The sites of deposition varied as the period progressed, for the warping and faulting of the surface, the igneous extrusions, and the deposition of sediments obliterated old basins and brought new ones into existence. The marine Tertiary formations are confined to the borders of the continent, appearing along the Atlantic, Gulf and Pacific coasts. The brackish water formations occur in some parts of the same general areas, while the terrestrial formations are found in and about the western mountains. As in other parts of the world, the chiefest palaeontological interest of the Tertiary attaches to the mammalian fossils.
The Eocene beds are unconformable, generally, upon the Cretaceous, and unconformable beneath the Miocene. On the Atlantic Eocene System. coast they are nearly horizontal, but dip gently seaward. On this coast they are nowhere more than a few hundred feet thick. In the Gulf region the system is more fully represented, and attains a greater thickness—1700 ft. at least. In the Gulf region the Eocene system contains not a little ​non-marine material. Thus the lower Eocene has some lignite in the eastern Gulf region, while in Texas lignite and saliferous and gypsiferous sediments are found, though most of the system is marine and of shallow water origin. The Eocene of the western Gulf region is continued north as far as Arkansas. The classification of the Eocene (and Oligocene) formations in the Gulf region, especially east of the Mississippi, is as follows:—
| 4. Jacksonian | | Upper Eocene. |
| 3. Claibornian | | Middle Eocene. |
| 2. Chickasawan | $\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,$ | Lower Eocene. |
| 1. Midwayan | $\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,$ | Lower Eocene. |
The Jacksonian is sometimes regarded as Oligocene. This classification is based almost wholly on the fossils, for there seems to be little physical reason for the differentiation of the Oligocene anywhere on the continent.
On the Pacific coast the marine Eocene lies west of the Sierras, and between it and the Cretaceous there is a general, and often a great, unconformity. The system has been reported to have a thickness of more than 7000 ft. in some places, and locally (e.g. the Pescadero formation) it is highly metamorphic. The Eocene of southern California carries gypsum enough to be of commercial value. It is also the source of much oil. The system is wanting in northern California and southern Oregon, but appears again farther north, and has great development in Oregon, where its thickness has been estimated at more than 10,000 ft. As in other comparable cases, this figure does not make allowance for the oblique attitude in which the sediments were deposited, and should not be construed to mean the vertical thickness of the system.
In Washington the Eocene is represented by the Puget series of brackish water beds, with an estimated thickness exceeding that of the marine formations of Oregon. Workable coal beds are distributed through 3000 ft. of this series. The amount of the coal is very great, though the coal is soft.
Terrestrial Eocene formations—eolian, fluvial, pluvial and lacustrine—are widespread in the western part of the United States, both in and about the mountains. By means of the fossils, several more or less distinct stages of deposition have been recognized. Named in chronological order, these are:—
1. The Fort Union stage, when the deposition was widespread about the eastern base of the northern part of the Rocky Mountains, and at some points in Colorado (Telluride formation) and New Mexico (Puerco beds), where volcanic ejecta entered largely into the formation. The Fort Union stage is closely associated with the Laramie, and their separation has not been fully effected.
2. The Wasatch stage, when deposition was in progress over much of Utah and western Colorado, parts of Wyoming, and elsewhere.
3. The Bridger stage, when deposition was in progress in the Wind River basin, north of the mountain of that name, and in the basin of Green river.
4. The Uinta stage, when the region south of the mountains of that name, in Utah and Colorado, was the site of great deposition.
More or less isolated deposits of some or all of these stages are found at numerous points in the western mountain region. The present height of the deposits, in some places as much as 10,000 ft., gives some suggestion of the changes in topography which have taken place since the early Tertiary. The thickness of the system in the west is great, the formations of each of the several stages mentioned above running into thousands of feet, as thicknesses are commonly measured.
The Miocene system, generally speaking, has a distribution similar to that of the Eocene. The principal formation of the Miocene System. Atlantic coastal plain is the Chesapeake formation, largely of sand. In Florida the system contains calcium phosphate of commercial value. The Miocene of the Atlantic and Gulf regions nowhere attains great thickness. The oil of Texas and Louisiana is from the Miocene (or possibly Oligocene) dolomite. On the Pacific coast the system has greater development. It contains much volcanic material, and great bodies of siliceous shale, locally estimated at 4000 ft. thick and said to be made up largely of the secretions of organisms. Such thicknesses of such material go far to modify the former opinion that the Tertiary periods were short. The Miocene of California is oil-producing. The terrestrial Miocene formations of the western part of the country are similar in kind, and, in a general way, in distribution, to the Eocene of the same region. The amount of volcanic material, consisting of both pyroclastic material and lava flows, is great.
At the close of the Miocene, deformative movements were very widespread in the Rocky Mountains and between the principal development of the Coast ranges of California and Oregon, and mountain-making movements, new or renewed, were somewhat general in the west. At the close of the period the topography of the western part of the country must have been comparable to that of the present time. This, however, is not to be interpreted to mean that it has remained unmodified, or but slightly modified since that time. Subsequent erosion has changed the details of topography on an extensive scale, and subsequent deformative movements have renewed large topographic features where erosion had destroyed those developed by the close of the Miocene. But in spite of these great changes since the Miocene, the great outlines of the topography of the present were probably marked out by the close of that period. Volcanic activity and faulting on a large scale attended the deformation of the closing stages of the Miocene.
The Pliocene system stands in much the same stratigraphic relation to the Miocene as the Miocene does to the Eocene. The marine Pliocene System. Pliocene has but trifling development on the Atlantic coast north of Florida, and somewhat more extensive development in the Gulf region. The marine Pliocene of the continent has its greatest development in California (the Merced series, peninsula of San Francisco), where it is assigned a maximum thickness of nearly 6000 ft., and possibly as much as 13,000 ft. This wide range is open to doubt as to the correlation of some of the beds involved. Thicknesses of several thousand feet are recorded at other points in California and elsewhere along the coast farther north. Marine Pliocene beds are reported to have an altitude of as much as 5000 ft. in Alaska. The position of these beds is significant of the amount of change which has taken place in the west since the Pliocene period. The non-marine formations of the Pliocene are its most characteristic feature. They are widely distributed in the western mountains and on the Great Plains. In origin and character, and to some extent in distribution, they are comparable with the Eocene and Miocene formations of the same region, and still more closely comparable with deposits now making. In addition to these non-marine formations of the west, there is the widespread Lafayette formation, which covers much of the Atlantic and Gulf coastal plain, reaching far to the north from the western Gulf region, and having uncertain limits, so far as now worked out, in various directions. The Lafayette formation has been the occasion of much difference of opinion, but is by many held to be a non-marine formation, made up of gravels, sands and clays, accumulated on land, chiefly through the agency of rain and rivers. Its deposition seems to have followed a time of deformation which resulted in an increase of altitude in the Appalachian Mountains, and in an accentuation of the contrast between the highlands and the adjacent plains. Under these conditions sediments from the high lands were washed out and distributed widely over the plains, giving rise to a thin but widespread formation of ill-assorted sediment, without marine fossils, and, for the most part, without fossils of any kind, and resting unconformably on Cretaceous, Eocene and Miocene formations. To the seaward the non-marine phase of the formation doubtless grades into a marine phase along the shore of that time, but the position of this shore as not been defined. The marine part of the Lafayette is probably covered by sediments of later age.
In earlier literature the Lafayette formation was described under the name of Orange Sand, and was at one time thought to be the southern equivalent of the glacial drift. This, however, is now known not to be the case, as remnants of the formation, isolated by erosion, lie under the old glacial drift in Illinois, and perhaps elsewhere. It seems probable that the Lafayette formation of the Gulf coastal plain is continuous northward and westward with gravel deposits on the Great Plains, washed out from the Rocky Mountains to the west. The careful study of these fluvial formations is likely to throw much light on the history of the deformative movements and changes in topography in the United States during the late stages of geological history.
Deformative movements of the minor sort seem to have been in progress somewhat generally during the Tertiary periods, especially in the western part of the country, but those at the close of the Pliocene seem to have exceeded greatly those of the earlier stages. They resulted in increased height of land, especially in the west, and therefore in increased erosion. This epoch of relative uplift and active erosion is sometimes called the Sierran or Ozarkian epoch. The details of the topography of the western mountains are largely of post-Pliocene development. The summits of some of the high mountains, such as the Cascades, appear to be remnants of a peneplain developed in post-Miocene time. If so, the mountains themselves must be looked upon as essentially post-Pliocene. Deformative movements resulting in closing folding were not common at this time, but such movements affected some of the coast ranges of California. This epoch of great deformation and warping marks the transition from the Tertiary to the Quaternary.
Quaternary Formations.—The best-known formations of the Quaternary period are those deposited by the continental glaciers Glacial. which were the distinguishing feature of the period and by the waters derived from them. The glacial drift covers something like half of the continent, though much less than half of the United States. Besides the drift of the ice-sheets, there is much drift in the western mountains, deposited by local glaciers. Such glaciers existed in all the high mountains of the west, even down to New Mexico and Arizona.
The number of glacial epochs now recognized is five, not counting minor episodes. Four defined zones of interglacial deposits are detected, all of which are thought to represent great recessions of the ice, or perhaps its entire disappearance. The climate of some of the interglacial epochs was at least as warm as that of the present time in the same regions. The glacial epochs which have been ​differentiated are the following, numbered in chronological order: (5) Wisconsin, (4) Iowan, (3) Illinoian, (2) Kansan, (1) Sub-Aftonian, or Jerseyan. Of these, the Kansan ice-sheet was the most extensive, and the later ones constitute a diminishing series.
Essentially all phases of glacial and aqueo-glacial drift are represented. The principal terminal moraines are associated with the ice of the Wisconsin epoch. Terminal moraines at the border of the Illinoian drift are generally feeble, though widely recognizable, and such moraines at the margin of the Iowan and Kansan drift sheets are generally wanting. The edge of the oldest drift sheet is buried by younger sheets of drift in most places.
Loess is widespread in the Mississippi River basin, especially along the larger streams which flowed from the ice. Most of the loess is now generally believed to have been deposited by the wind. The larger part of it seems to date from the closing stages of the Iowan epoch, but loess appears to have come into existence after other glacial epochs as well. Most of the fossils of the loess are shells of terrestrial gastropods, but bones of land mammals are also found in not a few places. Some of the loess is thought to have been derived by the wind from the surface of the drift soon after the retreat of the ice, before vegetation got a foothold upon the new-made deposit; but a large part of the loess, especially that associated with the main valleys, appears to have been blown up on to the bluffs of the valleys from the flood plains below. As might be expected under these conditions, it ranges from fine sand to silt which approaches clay in texture. Its coarser phases are closely associated with dunes in many places, and locally the loess makes a considerable part of the dune material.
Much interest attaches to estimates of time based on data afforded by the consequences of glaciation. These estimates are far apart, and must be regarded as very uncertain, so far as actual numbers are concerned. The most definite are connected with estimates of the time since the last glacial epoch, and are calculated from the amount and rate of recession of certain falls, notably those of the Niagara and Mississipi (St Anthony Falls) rivers. The estimate of the time between the first and last glacial epochs is based on changes which the earlier drift has undergone as compared with those which the younger drift has undergone. Some of the estimates make the lapse of time since the first glacial epoch more than a million years, while others make it no more than one-third as long. The time since the last glacial epoch is but a fraction of the time since the first—probably no more than a fifteenth or a twentieth.
Outside the region affected by glaciation, deposits by wind, rain, rivers, &c., have been building up the land, and sedimentation has Non-glacial. been in progress in lakes and about coasts. The non-glacial deposits are much like the Tertiary in kind and distribution, except that marine beds have little representation on the land. On the coastal plain there is the Columbia series of gravels, sands and loams, made up of several members. Its distribution is similar to that of the Lafayette, though the Columbia series is, for the most part, confined to lower levels. Some of its several members are definitely correlated in time with some of the glacial epochs. The series is widespread over the lower part of the coastal plain. In the west the Quaternary deposits are not, in all cases, sharply separated from the late Tertiary, but the deposits of glacial drift, referable to two or more glacial epochs, are readily differentiated from the Tertiary; so, also, are certain lacustrine deposits, such as those of the extinct lakes Bonneville and Lahontan. On the Pacific coast marine Quaternary formations occur up to elevations of a few scores of feet, at least, above the sea.
Igneous rocks, whether lava flows or pyroclastic ejections, are less important in the Quaternary than in the Tertiary, though volcanic activity is known to have continued into the Quaternary. The Quaternary beds of lakes Bonneville and Lahontan have been faulted in a small way since they were deposited, and the old shore lines of these lakes have been deformed to the extent of hundreds of feet. So also have the shorelines of the Great Lakes, which came into existence at the close of the glacial period.
Much has been written and more said concerning the existence of man in the United States before the last glacial epoch. The present state of evidence, however, seems to afford no warrant for the conclusion that man existed in the United States before the end of the glacial period. Whatever theoretical reasons there may be for assuming his earlier existence, they must be held as warranting no more than a presumptive conclusion, which up to the present time lacks confirmation by certain evidence.
The following sections from selected parts of the country give some idea of the succession of beds in various type regions. The thicknesses, especially where the formations are metamorphosed, are uncertain.
West Central Massachusetts
| Triassic. |
| Chicopee shale | 200 | ft. | (?) |
| Granby tuff | 580 | ” |
| Blackrock diabase (cones and dikes) |
| Longmeadow sandstone | 1000 | ” |
| Sugarloaf arkose | 4660 | ” |
| Mount Toby conglomerate |
| Unconformity. |
| Devonian. |
| Bernardston series | 1950 | ft. |
| Unconformity. |
| Silurian. |
| Leyden argillite | 300 | ft. |
| Conway schist $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,$   Amherst schist   Brinfield fibrolite-schist | 5000 | ” | (?) |
| Goshen schist | 2000 | ” | (?) |
| Unconformity. |
| Ordovician. |
| Hawley schist | 2000 | ft. | (?) |
| Savoy schist | 5000 | ” | (?) |
| Chester amphibolite | 3000 | ” | (?) |
| Rowe schist | 4000 | ” | (?) |
| Hoosic schist | 1500 | ” | (?) |
| Unconformity. |
| Cambrian. |
| Becket gneiss | 2000 | ft. | (?) |
| Unconformity. |
| Proterozoic. |
| Washington gneiss | 2000 | ft. | (?) |
| (Base not exposed.) |
The above section is fairly representative for considerable parts of New England.
West Virginia, &c.
| Pennsylvanian. |
| (Top of system removed by erosion.) |
| Braxton formation | 700 | ft. |
| Upshur sandstone | 300- 500 | ” |
| Pugh formation | 300- 450 | ” |
| Pickens sandstone | 400- 500 | ” |
| Unconformity. |
| Mississippian. |
| Canaan formation | 1000-1300 | ft. |
| Greenbrier limestone | 350- 400 | ” |
| Pocono sandstone | 70-  90 | ” |
| Devonian. |
| Hampshire formation | 1500-1800 | ft. |
| Jennings formation | 3000-3800 | ” |
| Romney shale | 1000-1300 | ” |
| Unconformity. |
| Monterey sandstone | 50- 200 | ft. |
| Silurian. |
| Lewiston limestone | 550-1050 | ft. |
| Rockwood formation | 100- 800 | ” |
| Cacapon sandstone | 100- 630 | ” |
| Tuscarora quartzite | 30- 300 | ” |
| Juniata formation | 205-1250 | ” |
| Ordovician. |
| Martinsburg shale | 800-1800 | ft. |
| Middle and Upper Cambrian. |
| Shenandoah limestone | 2400 | ft. |
| (Base not exposed.) |
This section is fairly representative for the Appalachian Mountain tract, though the Cambrian is often more fully represented.
Ohio
| Permian. |
| Dunkard formation | c. 25 | ft. |
| Pennsylvanian. |
| Monongahela formation | 200- 250 | ft. |
| Conemaugh formation | 400- 500 | ” |
| Alleghany formation | 165- 300 | ” |
| Pottsville conglomerate | 250 | ” |
| Unconformity. |
| Mississippian. |
| Maxville limestone | c. 25 | ft. |
| Waverley series— |
| Logan group | 100- 150 | ft. |
| Black Hand conglomerate | 50- 500 | ” |
| Cuyahoga shale | 150- 300 | ” |
| Sunbury shale | 5-  30 | ” |
| Berea grit | 5- 175 | ” |
| Bedford shale | 50- 150 | ” |
| Devonian. |
| Ohio shale | 300-2600 | ft. |
| Olentangy shale | 20-  35 | ” |
| Delaware limestone | 30-  40 | ” |
| Columbus limestone | 110 | ” |
| Silurian. |
| Monroe formation | 50- 600 | ft. |
| Niagara group | 150- 350 | ” |
| Clinton limestone | 10-  50 | ” |
| Medina shales (?) | 50- 150 | ” |
| (Belfast bed.) ​ |
| Ordovician. |
| Saluda bed | 20 ± | ft. |
| Richmond formation | 300 ± | ” |
| Lorraine formation | 300 ± | ” |
| Eden (Utica) shale | 250 | ” |
| Trenton limestone | 130 | ” |
Iowa
| Glacial drift. |
| Unconformity. |
| Upper Cretaceous. |
| Benton formation | 0- 150 | ft. |
| Dakota formation | 50- 100 | ” |
| Unconformity. |
| Pennsylvanian. |
| Missouri formation | 1500 | ft. |
| Des Moines formation | 250- 400 | ” |
| Unconformity. |
| Mississippian. |
| St Louis limestone | 100 | ft. |
| Osage (Augusta) formation | 200- 300 | ” |
| Kinderhook formation | 150- 200 | ” |
| Devonian. |
| Lime Creek formation | 80 | ft. |
| State Quarry beds | 20-  40 | ” |
| Sweetland Creek shales | 20-  40 | ” |
| Unconformity. |
| Cedar Valley limestone | 250- 300 | ft. |
| Wapsipinicon formation (Independence, Fayette, Davenport) | 100- 150 | ” |
| Silurian. |
| Anamosa limestone | 50-  75 | ft. |
| Le Claire limestone | 50 | ” |
| Delaware stage | 200 | ” |
| Unconformity. |
| Ordovician. |
| Maquoketa shales | 175 | ft. |
| Possible Unconformity. |
| Galena-Trenton limestone | 290 | ft. |
| St Peters sandstone | 100 | ” |
| Oneota formation (includes Shakopee, New Richmond and Oneota proper) | 300 | ” |
| Cambrian. |
| St Croix sandstone ( = Potsdam) | 1000 | ft. |
| Unconformity. |
| Proterozoic. |
| Sioux quartzite | (?) |
This section is fairly representative for much of the central Mississippi Basin.
Oklahoma
| Pennsylvanian. |
| (Summit removed by erosion.) |
| Seminole conglomerate | 50 | ft. |
| Holdenville shale | 260 | ” |
| Wewaka formation | 700 | ” |
| Wetumka shale | 120 | ” |
| Calvin sandstone | 145- 240 | ” |
| Senora formation | 140- 485 | ” |
| Stuart shale | 90- 280 | ” |
| Thurman sandstone | 80- 260 | ” |
| Boggy shale | 2000-2600 | ” |
| Savannah sandstone | 750-1100 | ” |
| McAlester shale | 1150-1500 | ” |
| Hartshorne sandstone | 150- 200 | ” |
| Atoka formation (Chickahoc chert lentil) | 3200 | ” |
| Wapanucka limestone | 100- 150 | ” |
| Mississippian. |
| Caney shale | 1500 | ft. |
| Devonian. |
| Woodford chert | 600 | ft. |
| Silurian. |
| Hunton limestone | 160 | ft |
| Sylvan shale (upper part) | 50- 100 | ” |
| Ordovician. |
| Sylvan shale (lower part) | 250 | ft. |
| Viola limestone | 750 | ” |
| Simpson series | 1600 | ” |
| Arbuckle limestone | 4000-6000 | ” |
| Cambrian. |
| Regan sandstone | 50- 100 | ft. |
| Unconformity. |
| Pre-Cambrian. |
| Tishomingo granite | (?) |
Composite section. The upper part is taken from vicinity of Coalgate, the lower part from the vicinity of Atoka.
West Central Colorado
| Eocene or later. |
| West Elk breccia | 3000 | ft. |
| Unconformity. |
| Cretaceous. |
| Ruby formation | 2500 | ft. |
| Unconformity. |
| Ohio formation (local only) | 200 | ft. |
| Unconformity. |
| Laramie formation | 2000 | ft. |
| Montana formation | 2800 | ” |
| Niobrara formation | 100- 200 | ” |
| Benton formation | 150- 300 | ” |
| Dakota formation | 40- 300 | ” |
| Jurassic. |
| Gunnison formation | 350- 500 | ft. |
| Unconformity. |
| Pennsylvanian. |
| Maroon conglomerate | 4500 | ft. |
| Possible unconformity. |
| Weber limestone | 100- 550 | ft. |
| Unconformity. |
| Mississippian. |
| Leadville limestone | 400- 525 | ft. |
| Apparent unconformity. |
| Ordovician. |
| Yule limestone | 350- 450 | ft. |
| Upper Cambrian. |
| Sawatch quartzite | 50- 350 | ft. |
| Unconformity. |
| Archean. |
The Bighorn Mountains of Wyoming
| Cretaceous. |
| De Smet formation (shale and sandstone) | 4000 | ft. |
| Kingsbury conglomerate | 0-1500 | ” |
| Piney formation (shale and sandstone) | 2500 | ” |
| Parkman sandstone | 350 | ” |
| Pierre shale | 1500-3500 | ” |
| Colorado formation | 1050-1700 | ” |
| Comanchean. |
| Cloverly formation (upper part may be Cretaceous) | 30- 300 | ft. |
| Morrison formation (may be Jurassic) | 100- 300 | ” |
| Jurassic. |
| Sundance formation | 250- 350 | ft. |
| Unconformity. |
| Triassic and Permian. |
| Chugwater formation | 750-1200 | ft. |
| Pennsylvanian. |
| Tensleep sandstone | 30- 150 | ft. |
| Amsden sandstone | 150- 350 | ” |
| Mississippian. |
| Madison limestone | 1000 | ft. |
| Unconformity. |
| Ordovician. |
| Bighorn limestone | 300 | ft. |
| Unconformity. |
| Cambrian (Upper). |
| Deadwood formation | 900 | ft. |
| Unconformity. |
| Pre-Cambrian. |
| Granites. |
This section is fairly representative for the Rocky Mountains.
Southern California
| Quaternary. |
| Alluvium, &c. |
| Terrace deposits and dune sand. |
| Pliocene (?) |
| Paso Robles formation | 1000 + | ft. |
| Unconformity. |
| Miocene (?) |
| Pismo formation (in south part of area) | 3000 ± | ft. |
| Santa Margarita (in north part of area) | 1550 ± | ” |
| Unconformity. |
| Miocene. |
| Monterey shale | 5000-7000 | ft. |
| Vaquero sandstone | 0- 500 | ” |
| Unconformity. |
| Cretaceous. |
| Atascadero formation | 3000-4000 | ft. |
| Unconformity. |
| Comanchean. |
| Toro formation (Knoxville) | 3000 ± | ft. |
| Unconformity. ​ |
| Jura-Trias. |
| San Luis formation (Franciscan) | 1000 ± | ft. |
| Unconformity. |
| Granite—age undetermined. |
This section is representative of the southern Pacific coast.
Section in Central Washington
| Pliocene (?). |
| Howson andesite | 250 | ft. |
| Miocene. |
| Keechelus andesite series | 4000 | ft. |
| Unconformity. |
| Guye formation (sedimentary beds with some lava flows) | 3500 ± | ft. |
| Eocene. |
| Roslyn formation (sandstone and shale; coal) | c. 3000 | ft. |
| Teanaway basalt | 4000 | ” |
| Kachess rhyolite | 0-2000 | ” |
| Swauk formation (clastic rocks with some tuff, &c.) | 200-5000 | ” |
| Unconformity. |
| Pre-Tertiary. |
| Igneous and metamorphic rocks. |
This section is representative of the north-west part of the country.
Bibliography.—A detailed bibliography for North American geology from 1732 to 1891, inclusive, is given in U.S. Geological Survey Bulletin 127 (1896); for 1892-1900 in Bulletin 188 (1902); for 1901-1905 in Bull. 301 (1906); for 1906-1907 in Bull. 372 (1909); for 1908 in Bull. 409 (1909), &c. A few of the more important and available publications are enumerated below.
General Treatises.—T. C. Chamberlin and R. D. Salisbury, Geologic Processes (New York) and Earth History (2 vols., New York); J. D. Dana, Manual of Geology (New York, 1862); W. B. Scott, Introduction to Geology (New York, 1897); and Joseph Le Conte, Elements of Geology (New York, 1878).
Official Reports.—F. V. Hayden, Reports of the U.S. Geological and Geographical Survey of the Territories (12 vols., Washington, 1873-1883); Clarence King, Geological Exploration of the Fortieth Parallel (7 vols. and atlas, Washington, 1870-1880); George M. Wheeler, Geographical and Geological Exploration and Surveys West of the 100th Meridian (7 vols. and 2 atlases, Washington, 1877-1879); and Reports of the U.S. Geological Survey (since 1880): (1) Monographs on special topics and areas, about 50 in number; (2) Professional Papers—monographic treatment of somewhat smaller areas and lesser topics, about 60 in number; (3) Bulletins, between 300 and 400 in number; and (4) Annual Reports (previous to 1903) containing many papers of importance, of the sort now published as Professional Papers. Reports of state geological surveys have been published by most of the states east of the Missouri river, and some of those farther west (California, Washington, Kansas, Nebraska and Wyoming) and south (Arkansas, Texas and Louisiana). Among the more important periodicals are the Bulletin of the Geological Society of America (Rochester, N.Y., 1889 seq.); the American Journal of Science (New Haven, Conn., 1818 seq.); the American Geologist (Minneapolis, 1888 seq.); Journal of Geology (Chicago, 1893 seq.); Economic Geology (Lancaster, Pa., 1905 seq.). Occasional articles of value are to be found in the American Naturalist and Science, and in the Transactions and Proceedings of various state and municipal academies of science, societies, &c.  (R. D. S.; T. C. C.)