# A Treatise on Geology/Chapter 4

CHAP IV.
BY following the methods previously described (pages 18, 19.) the whole series of strata existing in any country can he known; by comparing the results thus obtained in different countries, the extent of the strata, and the degree of generality of the causes concerned in producing them, can also be known. The investigations, in both respects, have now proceeded so far as to fully justify a geologist in asserting, that the principal features of the stratified rocks in the crust of the globe are very similar over large regions; the aggregate thickness of their mass is everywhere limited to a few miles; the order of succession among the principal groups is the same, or analogous; even the minute variations of their composition, aggregation, and structure are observable in remote situations; their organic contents are reducible to the same schemes of classification, and everywhere indicate several great physical changes on the surface of the globe, since it became the theatre of vegetable and animal life.
The foundations of all sound generalisations in geological science are accurate and mutually explanatory sections and maps of the whole series of stratified and igneous rocks existing in each natural geological district; a term by which we wish to express a part of the earth's crust, whether large or small, in which the formation of aqueous deposits has followed, amidst many local irregularities, one general law of succession. Such sections and maps express, by one common type or formula, the general result of many separate and local investigations; the principal local deviations from the general type must be on no account omitted, for these ​limited differences are often more important in theory (as well as in practical applications) than all the general resemblances. Assuming that the British islands form such a natural district, we shall be able to present a satisfactory general table or section of the series of strata which here compose the crust of the globe, placed in the order of their succession downwards, from the surface of the most recent aqueous deposits.
Table of British Deposits.
Superficial Accumulations. :
| Depositions from springs, rivers, glaciers, lakes, the sea, under ordinary circumstances. |||||
| Depositions from some of these agents under extraordinary circumstances. |||||
| Stratified Rocks. |||||
| Tertiary or Cainozoic Strata. |||||
| Names of formations. | Thickness in yards. | Thickness in yards. | Subdivisions or groups. | Nature of the deposits. |
| Crag. | 16 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \end{matrix}}\right.$ | Upper or mammaliferous crag. | Marine and estuary shells, pebbles, bones, sand, &c. |
| Crag. | 16 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \end{matrix}}\right.$ | Middle or red crag. | Marine shells, pebbles, bones, sand, &c. |
| Crag. | 16 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \end{matrix}}\right.$ | Lower or coralline crag. | Marine shells and corals in sand, or coarse limestone. |
| Freshwater marks. | 33 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Upper freshwater. | Marly limestone and clays. |
| Freshwater marks. | 33 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Estuary beds. | Marine or estuary clays, marls, &c. |
| Freshwater marks. | 33 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Lower freshwater. | Marly limestone and clays. |
| London Clay. | 200 to 600 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \end{matrix}}\right.$ | London clay (upper). | Clay with septaria, &c. |
| London Clay. | 200 to 600 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \end{matrix}}\right.$ | Sand and clays. | Coloured sands, clays. |
| London Clay. | 200 to 600 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \end{matrix}}\right.$ | London clay (lower). | Clay with septaria, &c. |
| London Clay. | 200 to 600 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \end{matrix}}\right.$ | Plastic clay. | Variegated sands, clays, lignite, &c. |
| Secondary or Mesozoic Strata. |||||
| Cretaceous System. |||||
| Chalk. | 200 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Upper chalk. | Soft chalk, with flints in layers. |
| Chalk. | 200 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Lower chalk. | Harder chalk. |
| Chalk. | 200 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Chalk marl. | Soft argillaceous chalk. |
| Green Sand. | 160 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Upper green sand. | Green sands. |
| Green Sand. | 160 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Gault. | Blue marl or clay. |
| Green Sand. | 160 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Lower green sand. | Ferruginous, brown, or green sand, with local deposits of limestone. ​ |
| Oolitic System. |||||
| Names of formations. | Thickness in yards. | Thickness in yards. | Subdivisions or groups. | Nature of the deposits. |
| Wealden. | 300 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Weald clay. | Clays and calcareous layers. |
| Wealden. | 300 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Hastings sands. | Variously coloured sands and clays. |
| Wealden. | 300 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Purbeck beds. | Clays and limestones. |
| Upper oolite. | 130 | $\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.$ | Portland oolite. | Limestone, often cherty, with sand. |
| Upper oolite. | 130 | $\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.$ | Kimmeridge clay. | Blue clay, with septaria. |
| Middle oolite. | 150 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Upper calcareous grit. | Sandstone (calcareous). |
| Middle oolite. | 150 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Coralline oolite. | Oolitic limestone. |
| Middle oolite. | 150 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Lower calcareous grit. | Sandstone (calcareous). |
| Middle oolite. | 150 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Oxford clay. | Blue clay, with septaria. |
| Middle oolite. | 150 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Kelloway rock. | Sandstone (calcareous). |
| Lower oolite. | 130 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Cornbrash. | Coarse limestone. |
| Lower oolite. | 130 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Forest marble. | Coarse limestone, sands, and clays. |
| Lower oolite. | 130 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Great oolite. | Limestone, oolitic, compact or sandy. |
| Lower oolite. | 130 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Fullers' earth. | Limestones, clays, &c. |
| Lower oolite. | 130 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Inferior oolite. | Limestone, oolitic, ferruginous. |
| Lower oolite. | 130 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Sand. | Calcareous or ferruginous sand and sandstone. |
| Lias. | 350 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Upper lias shale. | Blue laminated clay. |
| Lias. | 350 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Marlstone. | Sandy, calcareous, and irony beds. |
| Lias. | 350 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Middle lias shale. | Blue laminated clay. |
| Lias. | 350 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Lias limestone. | Blue and white compact limestones. |
| Lias. | 350 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Lower lias marls. | Clays of different colours. |
| Saliferous or New Red Sandstone System. |||||
| New red sandstone. | 300 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Variegated clays. | Red, greenish, &c. clays, with gypsum. |
| New red sandstone. | 300 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Keuper. | Sandstones, usually light-coloured, with plants, and vertebrata. |
| New red sandstone. | 300 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Red clays. | Red clays, with a few pale bands, gypsum, salt, &c. |
| New red sandstone. | 300 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Sandstone. | Red and white sandstone, with bands of clay. |
| New red sandstone. | 300 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Conglomerate. | Sand and sandstone, with pebbles of quartz, porphyry, &c. ​ |
| Primary or Palœozoic Strata. |||||
| Magnesian or Permian System. |||||
| Names of formations. | Thickness in yards. | Thickness in yards. | Subdivisions or groups. | Nature of the deposits. |
| Magnesian limestone (North of England). | 100 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Knottingley limestone. | Grey laminated limestone. |
| Magnesian limestone (North of England). | 100 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Gypseous marls. | Red and white clays, &c. |
| Magnesian limestone (North of England). | 100 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Magnesian limestone. | Yellow, granular, &c. limestone. |
| Magnesian limestone (North of England). | 100 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Marl slate. | Laminated calcareous beds, with fishes. |
| Magnesian limestone (North of England). | 100 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Rothetodteliegende. | Red sandstones and clays. |
| Carboniferous System. |||||
| Coal. | 1000 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | The subdivisions are only locally ascertained. | Strata of sandstone, shale, ironstone, with rare deposits of freshwater limestone. |
| Coal. | 1000 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Millstone grit | Sandstones, often coarse grained, or pebbly; shales, iron stones, thin limestones. |
| Carboniferous limestones (N. of England). | 900 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Yoredale rocks. | Limestones, sandstones, shales, coals. |
| Carboniferous limestones (N. of England). | 900 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Sear limestones. | Limestones. |
| Carboniferous limestones (N. of England). | 900 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Alternating sandstone, and limestones. | Limestones, sandstones, &c., often red. |
| Old Red or Devonian System. |||||
| Old red sandstone (Herefordshire.) | 100 to 3300 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Conglomerate group | Conglomerates and sandstones. |
| Old red sandstone (Herefordshire.) | 100 to 3300 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Cornstone group. | Coloured clays and concretionary limestones. |
| Old red sandstone (Herefordshire.) | 100 to 3300 | $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ | Tilestone group. | Flagstones and clays. The lower beds pass to the group below. ​ |
| Silurian System. |||||
| U p p e r S i l u r i a n. $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ Ludlow rocks. Wenlock rocks. | 660 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Upper Ludlow rock. | Laminated sandstone. |
| U p p e r S i l u r i a n. $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ Ludlow rocks. Wenlock rocks. | 660 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Aymestry limestone. | Subcrystalline limestone. |
| U p p e r S i l u r i a n. $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ Ludlow rocks. Wenlock rocks. | 660 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Lower Ludlow rock. | Sand shale, with concretionary limestone. |
| U p p e r S i l u r i a n. $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ Ludlow rocks. Wenlock rocks. | 600 | $\left\{{\begin{matrix}\ \ \\\ \end{matrix}}\right.$ | Wenlock limestone. | Grey and blue subcrystalline limestonne. |
| U p p e r S i l u r i a n. $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ Ludlow rocks. Wenlock rocks. | 600 | $\left\{{\begin{matrix}\ \ \\\ \end{matrix}}\right.$ | Wenlock shale. | Shale and earthy limestone. |
| L o w e r S i l u r i a n. $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ Wenlock rocks. Llandeilo rocks. | 830 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Limestones and sandstones. | Laminated limestones and sandstones. |
| L o w e r S i l u r i a n. $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ Wenlock rocks. Llandeilo rocks. | 830 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | Conglomerates, &c. | Gritstones, conglomerates, limestones. |
| L o w e r S i l u r i a n. $\left\{{\begin{matrix}\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \end{matrix}}\right.$ Wenlock rocks. Llandeilo rocks. | 400 | $\left\{{\begin{matrix}\ \\\ \\\ \end{matrix}}\right.$ | | Dark calacareous flags, sandstones, &c. |
Cambrian System. (See page 124.)
Hypozoic Strata. (See page 111.)
Granite and other unstratified Rocks, the effect of Heat.
In possession of this complete section of all the principal masses of stratified rocks in the British isles, and guided by a map of the ranges of each of these on the surface,—aware, also, that within the narrow compass of these islands some of the groups of strata vary extremely (as the lower oolites, which are principally calcareous near Bath, but principally arenaceous near Whitby), and others have only a limited range (as the magnesian limestones), we may proceed to inquire how far the sections of other natural districts agree with that given above.
Throughout the great basins of Europe, and parts of Asia and Africa, including the countries bordering on the German Ocean, the Baltic, the Black Sea, and the Mediterranean, within the mountain boundaries of the Ural, Caucasus, Greece, Calabria, the Atlas, Western Spain, Brittany, Cornwall, the west of Ireland, Scotland, and Scandinavia, the general features and succession ​of the great masses are the same: most of the systems of strata are also to be recognised, either in the mountains or the lower ground; and fresh additions to these analogies are continually added by geological travellers. But when we come to consider the constituent formations, discordance is manifested of the same kind as that which, as before observed, appears between different parts of England with respect to certain oolitic and carboniferous formations.
In Scandinavia, as well as along the Alps, and Grampians, hypozoic strata are predominant, while mica schist and gneiss are rare in the Harz, Cornwall, and Wales. There is more carboniferous limestone in England and Ireland than in all Europe besides. The oolites of Germany and France sometimes perfectly resemble, in composition and succession, that group in England; but on the Italian side of the Alps, and in Greece, they have different characters. The chalk formation is little seen about or beyond the Alps; and in the Carpathians green sand appears in plenty, but little or no chalk. Turning to more distant localities, we find, in North America, a vast series of widely spread stratified deposits, much allied to those of Europe, grouped, for the most part, in similar systems; but the series between the cretaceous and carboniferous rocks is much less developed than in Europe. On the contrary, in the Himalaya mountains, and the basin of the Indus, these formations are greatly developed, and rocks of the lias and oolitic formations are perfectly identified. As a general result, it appears already ascertained that the same great divisions of strata may be applied to nearly all parts of the globe; that, even in very distant localities, the same systems of strata were produced, though sometimes in isolated patches; but that the particular formations, though often very extensively spread, are yet somewhat irregular in their expansion, some extending in one direction, and others in a different one, so as clearly to evince their dependence on local and variable conditions. ​
Varieties of Stratification.
Stratified rocks are either of equal thickness over a large extent of country, or attenuated to a wedge shape in some one direction, or decreasing in thickness every way from a certain point or district, so as to constitute a lenticular formation. Strata of one certain kind of rock, which are, in some places, accumulated into a uniform mass, become divided in other districts, and separated into distinct members by the interposition of wedge-shaped deposits. All these circumstances are represented in the annexed diagram (fig. 18.), where
limestone strata are marked l, shale beds s, gritstone g; the gritstone and shale being in lenticular or wedge shaped, and limestone in parallel, beds, divided in one direction, but conjoined in the other.
By observations of this kind in certain districts (e. g., in the north of England, along the Penine chain, and on the Yorkshire coast), it has been inferred, that the different strata of limestone, shale, and grit, have originated under different circumstances; the former being an oceanic deposit, but the two latter substances derived from the waste of ancient lands bordering on the sea in which the limestone was formed. This conclusion is strongly corroborated by the fact, that it is chiefly or wholly in gritstones and shales that land plants occur, while the marine exuviæ of shells, corals, &c. abound almost exclusively in the limestones.
The term stratum or layer is of general signification, and independent of the absolute thickness of the mass. ​By some writers (Dr. Smith and others) it has been used to express the whole of one mass of layers or beds of the same or nearly the same quality (as the Bath oolite), or one similar series of alternations (as the Weald clay). By others (Playfair, &c) it is applied to the thinner layers of rocks, which Smith denominates beds. Neither mode is perhaps inaccurate, yet it is convenient now to settle the nomenclature we must employ in the following descriptions.
Many rocks, as limestones, are divided by parallel, or nearly parallel, seams, into what are by the quarrymen called beds or posts; in some cases these are further divisible into laminæ. Moreover, it is the custom of geologists to include several rocks which are generally concomitant, and have some common characters of deposition and organic remains, under one title, viz. formation. The subjoined diagram (fig. 19.) will illustrate the use of these terms.
| | | Formation if Bath oolite above. |
| Lias formation | $\left\{{\begin{matrix}\ \\\\\\\\\\\\\\\\\\\\\\\\\\\ \end{matrix}}\right.$ | | Upper lias shales. |
| Lias formation | $\left\{{\begin{matrix}\ \\\\\\\\\\\\\\\\\\\\\\\\\\\ \end{matrix}}\right.$ | | Marlstone. |
| Lias formation | $\left\{{\begin{matrix}\ \\\\\\\\\\\\\\\\\\\\\\\\\\\ \end{matrix}}\right.$ | | Middle lias shales. |
| Lias formation | $\left\{{\begin{matrix}\ \\\\\\\\\\\\\\\\\\\\\\\\\\\ \end{matrix}}\right.$ | | Lias limestones. |
| Lias formation | $\left\{{\begin{matrix}\ \\\\\\\\\\\\\\\\\\\\\\\\\\\ \end{matrix}}\right.$ | | Lower lias shales. |
| | | Formation if Bath oolite above. |
Formation of Bath oolite above. Lias formation. Upper lias shales. Marlstone. Middle lias shales. Lias limestones. Lower lias shales. Formation of red sandstone below.
The shales in this diagram (a c e) are from 20 to 300 feet thick, and are composed of laminæ parallel to the planes of stratification; the limestones (b) are thin ​bedded, the beds being separated by thin clays: the marlstone series (d) consists of sandstone beds, calcareous beds, and ironstone bands, separated by thin clays or shales.
The lias formation is included between the Bath oolite formation above, and the red sandstone formation below.
According to this mode of description, the word stratum need never be used as a special term of definition, but reserved for general reasoning. The word series is found to be extremely serviceable in designating a number of similar or similarly associated rocks: the arbitrary word group is also convenient in geological description.
The lamination of rocks offers some interesting facts. Some beds of gritstone (as a) are composed of laminæ parallel to the plane of the beds; such lamination is generally produced by the alternation of mica, whose broad plates cause a partial disunion of the parallel laminæ of quartzose grains. Other beds (as b) are composed of oblique or curved laminæ, a circumstance generally
dependent on the irregular admixture of pebbles shells, or particles of unequal magnitude. The former may be supposed to be tranquil, the latter disturbed, deposits.
In shales and other argillaceous rocks, nodules of ironstone or limestone, aggregated round some solid
bodies (as a leaf or shell), are frequently included, and sometimes these interrupt the lamination of the ​shale, as in fig. 21. Such nodules are frequently traversed by plates of calcareous spar, and these receive the name of septaria.
In limestone beds the nodules of chert, in chalk the nodules of flint, often appear to have been aggregated round some previously solidified sponge, coral, or shell, This process of accretion round a nucleus is beautifully exemplified in certain "oolitic" limestones, so called from their being composed of spherical grains. Each of these consist of several concentric coats, collected round a previously solidified body, as a minute grain of sand, fragment of shell, or other centre of attraction. Radiating fibres frequently cross the spherical shells.
Something of this concretionary structure appears in particular unstratified rocks (as pitchstone); but for the most part the appearances previously described belong to the stratified formations exclusively.
Divisional Structures.
All rocks are traversed by divisional planes of less or greater width and frequency, and thus divided into masses of definable shapes and proportions. These "joints," as they are often called, present themselves under a great variety of appearances, but almost always such as to be intelligible on the supposition of the mass of the rock having been contracted, so as to separate into prismatic and other forms, as clay, starch, &c. contract and split by drying.
The joints vary in their combination, so as to produce masses of different forms in rocks of different nature: they also vary in rocks of the same nature which are of different antiquity: their frequency and regularity also depend upon the mineral aggregation of the rock: it is further probable that they are somewhat complicated with additional fissures, near axes and centres of elevation or depression of rocks. ​
Among unstratified rocks the most remarkable and best known form is that of the divided prisms of
basalt, as seen in Staffa and the Giant's Causeway. This appearance arises from the intersection of planes rectangulated to the surface of the rock, and meeting one another in several directions, so as to insulate polygonal prisms. These prisms are divided across, by concavo-convex surfaces, as fig. 23.
Much more common in these rocks is the form of an irregular polygonal prism not divided across, as in the greenstone and pitchstone of Corygills, Arran. It is interesting to observe, in vertical dykes of these rocks (as in Cleveland, Yorkshire), the prisms lying horizontally, and in other cases curved (as in Staffa), in obedience to the general law of the planes of the prismatic faces being at right angles to the bounding surface of the mass.
Many rocks of igneous origin (as greenstone, claystone, porphyry, sienite,) show this prismatic structure more or less distinctly, but none so perfectly as basalt.
A prismatic form of the masses is found also among stratified rocks, when these are very thick and of uniform composition, as in the rock-salt mines of Northwich (observed 1827), in the gypsum quarries of Montmartre, and in the thick scar limestone of Wharf dale (observed 1834).
A great variety of other appearances are presented in the stratified rocks by the various directions and intersections of the different sorts of joints.
Under particular circumstances, and especially in the vicinity of faults, anticlinal axes, and other forms of displacement, the beds of rock are frequently cracked in their substance; sometimes these cracks are filled with sparry substances (carbonate of lime frequently, metallic matters rarely); sometimes they are very minute chinks lined on the sides with dendritical oxide of iron or manganese, in which case they are called dry cracks. ​Their direction is very irregular, and there is no doubt that in many cases they are the effect of mechanical strain or tension in the mass of rocks which accompanied the displacement of the rocks. Near the anticlinal axes of Ribblesdale, sparry cracks are wonderfully numerous; but away from these axes the level beds are little marked by such accidents.
Besides these irregular cracks (c), which often do not pass through the whole mass of a bed, are joints (j) which divide at least one bed, and often several, and which exhibit some regularity of direction; these are so situated in the different beds, have such diversity of slopes, irregularity of number, openness, and other characters, and are so abundant in situations far from lines and points of displacement, as to leave no doubt that they are due to a very general cause.
Amongst these joints, some more open and extended, than others, passing through a greater number of beds, dividing a whole rock, or even a considerable portion of a formation, may be distinguished as fissures (f) or master joints. The diagram, fig. 24., is intended to convey a correct notion of these several divisional planes.
Viewed on a horizontal plan, joints frequently end in fissures; and these latter commonly exhibit a great degree of local symmetry. In the mountain limestone districts of the north of England, the arrangement of the fissures has been ascertained to be correctly ​represented by the following diagram, in which the breadth of shade in any direction corresponds to the number of fissures observed. Thus, in the direction N.N.W. and S.S.E., and E.N.E. and W.S.W., and about these lines, a greater number of fissures occur, thus producing two principal systems of divisions in the rocks at right angles to each other; while in the lines N.E. by N. and N.W. by W., and about these lines, few or no fissures have been noticed. Thus there are positive and negative axes of frequency and rarity of the fissures situated at right angles to each other respectively. The same result of predominance of fissures in N.N.W. directions is found to obtain in Derbyshire (Hopkins), in Cornwall (De la Beche), and in some districts of Ireland (Griffith).
The effect of these fissures in causing lines of weakness of the rocks may be understood from the diagram: the breadth occupied in each radius being proportioned ​to the number of long joints, or fissures, observed in that direction.
The following is the table of observations referred to.
GENERAL TABLE OF RESULTS FOR THE SECONDARY ROCKS OF YORKSHIRE. :
| Names of formation | No. of Obser- vations in Yorkshire | W. by N. | W. N. W. | N. W. by W. | N. W. | N. W. by N. | N. N. W. | N. by W. | N. | N. by E. | N. N. E. | N. E. by N. | N. E. | N. E. by E. | E. N. E. | E. by N. | E. |
| Magnesian limestone | 4 | | | | 1 | | 1 | | 1 | | 1 | | | | | | |
| Coal | 3 | | | | | | 1 | 1 | 1 | | | | | | | | |
| Millstone grit | 13 | | | | | 1 | 5 | 2 | | | | | | | 4 | 1 | |
| Chert group | 17 | | | | 1 | 4 | 6 | | | | | | 1 | | 4 | 1 | |
| Yordale Series | 35 | | 2 | | 1 | 1 | 8 | 5½ | 5½ | | | | 2 | 1 | 3 | 2 | 4 |
| Lower limestone | 15 | 1 | | | 4 | 1 | 2 | 1 | 2 | | | | | 1 | 1 | | 2 |
| Red sandstone | 1 | | | | | | | | | 1 | | | | | | | |
| Whin sill | 1 | | | | | | | | | | | | | | | | 1 |
| | 89 | 1 | 2 | 0 | 7 | 7 | 23 | 9½ | 9½ | 1 | 1 | 0 | 3 | 2 | 12 | 4 | 7 |
It appears that some remarkable differences of characters belong to the joints and fissures in rocks of different chemical and mineral quality. In limestone the joints are usually rectangled to the planes of stratification and frequently open and regular; in gritstone they are very irregular, but often widely open; while in argillaceous rocks they are usually much more numerous, but far less open, and often oblique to the planes of stratification. In conglomerate rocks there are few regular joints, but the rude fissures are sometimes remarkably large.
On considering the occurrence of joints with reference to the age of the rocks, it appears quite certain that it is among the older rocks that joints are most numerous and symmetrical. If we compare in this respect the old argillaceous slate, to the shale of a coal tract, and then with the clays of an oolitic district, or make a similar comparison of the ancient primary ​limestone of the highlands, with the calcareous rocks of later production, this dependence of the frequency and regularity of joints, on the age of the rocks, will clearly appear.
Cleavage.
Among the argillaceous slate rocks, a further peculiarity of internal structure takes place, which is deserving of special attention, since it appears to be the case of divisional planes carried to extreme in number and symmetry.
This structure, commonly called cleavage, is really distinct from joints and stratification, and may be, perhaps, understood in its relation to them by the accompanying sketch.
In this drawing, S is a plane of stratification dipping in the direction Δ; c c are the edges of planes of cleavage, which in the plane S continue in lines c′ c′. These planes are continuous, and very numerous in the fine ​grained beds s, which alternate with the coarse beds g g, but in these latter the laminæ of cleavage are often totally absent, j is a joint which varies its angle of dip in the different beds of rock. The line l l, at right angles to the dip of the strata Δ, is called the strike of the bed, and is of course level; and it is frequently observed that the horizontal line, or strike, of the cleavage, coincides with the strike of the strata. The planes of cleavage generally approach toward the perpendicular, whatever may be the amount of the dip of the strata: their course is almost exactly the same over immense spaces of country (in North Wales, in Cumberland, Charnwood Forest, &c.), and it is to them that the valuable substance called slate is owing. It is quite certain, in some instances, that this beautiful structure of the slate rocks was caused since the strata of these rocks were placed in their disturbed directions, and that it is the fruit of a peculiar degree of crystalline action in the mass; for in some cases at Aberystwith and elsewhere, the nearly vertical laminæ of cleavage cross highly contorted beds of slate dipping in various directions.
There are reasons for conjecturing that this cleavage of many of the argillaceous strata is an effect due to the pervading agency of heat; amongst others we may mention the fact that near igneous rocks (as at Coley Hill near Newcastle) something of the same kind is produced in shales of later date; and that among the Alps of Savoy, the lias clays are so altered near the axis of elevation, as to assume much of the aspect of an old slate country. The relation already pointed out between the strikes of cleavage and axes of elevation, leads to the supposition that pressure may be concerned in the result. Mr. Fox's experiments have given some countenance to the opinion that electrical currents have re-arranged the molecules in slate.
* ↑ As seen near Bath. In other parts of England it offers important differences, as will appear hereafter.
* ↑ So named by Murchison, from a large Russian district, in which he has found it more fully developed than in England. (Geol. of Russia.)
* ↑ As seen in Herefordshire, &c. In North and South Devon the series is different, as will be explained hereafter.
* ↑ As originally given by Murchison. (Sil. Syst. 1836.)
* ↑ Geology of Yorkshire, vol. ii. p. 9.
* ↑ For suggestions to observers on the subject of cleavage and joints, and a method of calculation to be applied to cases of inclined strata, see Guide to Geology, 3d edition. The Geological Intersector, a small and cheap instrument, which has been constructed by the author, and engraved by Mr. Lowry, may be used to represent the phenomena, and save the trouble of calculation.