# A Treatise on Geology/Chapter 3

GENERAL TRUTHS CONCERNING THE STRUCTURE OF THE EXTERNAL PARTS OF THE GLOBE.
FROM these facts and reasonings concerning the nature and constitution of the materials of the globe, derived from chemical and physical science, we may turn to contemplate the general truths obtained by direct processes of observation and induction, concerning the mode of arrangement of these materials, in that limited portion of the earth's mass which it is possible for man to explore by artificial excavations, or to understand by skilful interpretation of the disclosures effected by nature.
Beginning at the surface, and passing gradually towards the deeper parts, we shall be able easily to gather clear ideas of this fundamental portion of positive geology, without a right knowledge of which the otherwise pleasing task of following and examining the common reasonings in the science would be useless, if not presumptuous.
Structure of the External Parts of the Globe.
SOIL, the external investment of the land, though it somewhat veil from geologists the objects of their peculiar research, merits attention; for this thin covering varies in some real relation to the rocks beneath, and appears, in many instances, to be nothing else than the substance of those rocks decomposed by time, and altered by vegetable admixtures. The depth of soil is extremely irregular,—some feet thick over certain sandy rocks, a foot thick over clays, only a few inches ​thick over chalk. In valleys the soil is accumulated from the waste of the hill sides: the surface of many (especially primary) mountainous regions is devoid of soil. In particular districts the soil is not merely formed by decomposition of the rocks beneath; it contains sand, pebbles, &c., brought from a distance, either by actual streams or some extraordinary force of water. Thus, in many instances, a mixture of substances takes place very beneficial to the fertility of the soil.
Beneath this thin and irregular layer, in some countries, the solid rock appears; but in others masses of loose sands, clays, gravel, &c., are found, which lie in hollows, or on the surfaces of the subjacent rocks, 10, 50, 100, or more feet in depth. These have evidently been drifted by water, and deposited from it, but yet they do not properly enter into the structure of the crust of the earth, but must be viewed as superficial and local accumulations, produced under circumstances considerably different from those which determined the formation of rocks. To these accumulations the names of alluvial and diluvial deposits have been applied: it appears desirable at present to use, for them and the soil collectively, the term superficial accumulations.
Rocks, and the substances which they enclose, lie beneath the superficial accumulations, and constitute the crust of the earth as known to geologists. The term "rocks" is apt to mislead beginners; for under this title geologists rank clay, sand, coal, and chalk, as well as limestone, granite, slate, and basalt, and other hard and solid masses, to which the use of the term is generally restricted: and they do so because they are all and equally constituent parts of the crust of the earth; and this crust is generally of a rocky consistence. The embarrassment which may be felt from this unusual employment of the term will diminish as we proceed, and find ourselves led to adopt various other modes of designating, in detail, the masses which it will yet often be convenient to speak of together under the vague term of Rocks. ​
Forms of Rock Masses.
On mountain sides, in ravines, and sea cliffs, the rocky masses of the earth are exhibited free from the obscurity of superficial accumulations: the industry of man, in mines, wells, roads, canals, has added to the facilities granted by nature, and from these opportunities the structure of the crust of the earth, the arrangement and relative position of the rocks, are known in the most essential points. The different sorts of rocks are by no means mixed together in confusion, but placed in a regular and ascertained method of occurrence, and often arranged in a certain determinate order of succession. Almost all rocks exhibit to the careful observer some interesting circumstances of interior structure,—particular divisions of their substance by joints, cleavage, &c.; but, neglecting for the present these subjects, we shall fix our attention on the form of the rock masses taken in their totality.
A very large proportion of rocks are formed so as to spread over areas of 10, 100, or 1000 square miles, with thicknesses of only as many or fewer feet or even inches: these are said to be stratified (s), or formed like a stratum or layer. Fissures, dividing particular rocks, are sometimes filled up with another sort of rock, which is then said to appear as a dyke (d); various spars, metallic matters filling fissures, or embodied in the rocks, are called veins (v); and many rocks, neither stratified nor in the form of dykes or veins, are in this sense amorphous, but are generally ranked with dykes, veins, &c., as unstratified rocks (u).
Dykes and veins form but a small part of the mass of the crust of the globe, which consists principally of ​stratified and unstratified rocks. In the plains, and comparatively low portions of the earth, the rocks are almost universally stratified, the strata being often very thin, even to inches, but sometimes many yards or fathoms in thickness. The superficial area, over which a particular stratified rock expands, is sometimes enormous, chalk, for instance, has a range of many hundred miles in length, by 5, 10, 20, or more in width, in England and France, but sometimes very limited, as the magnesian limestone of the north of England, which ranges from Shields to Nottingham.
In more elevated districts, and on the flanks of mountainous regions, the rocks are also seen to be distinctly stratified. By patient attention it will be found that, even in the very midst of chains and groups of mountains, the marks of stratification may often be perceived; but it almost always happens that the axis of such chains or groups is formed by unstratified rocks, and these sometimes appear in lower situations.
Position of Rocks with respect to the Surface of the Earth.—Declination of Strata.
Stratified rocks have usually a nearly constant thickness, or else vary in this respect by insensible and regular gradation; their surfaces, or the planes of stratification, are therefore in general sensibly parallel, and their position may be known with respect to the surface of the earth, by observing the bearing of a level line (or strike) in the plane of stratification, and the angular amount of the descending slope (dip) or ascending slope (rise). The result of very numerous trials proves that the strata are over large surfaces often nearly but seldom quite horizontal; they dip, in fact, below the horizon, pass under the surface, and are covered up by other strata which also mostly clip in the same direction. Thus the surface of the earth in regions where stratified rocks occur is formed partly on their edges; and a section ​or vertical cut to some depth from the surface would present on its sides the appearance of the diagram, No. 6.
We may say that three fourths of the surface of the dry land of the globe is thus formed on the edges of moderately dipping strata: in all large districts the dip is found to be variable in amount and in direction, but, viewed on the great scale, always in harmony with one general law, which may be thus expressed:
The strata dip from the chains and groups of mountains under the plains which surround or divide them. Thus, from the group of Cumbrian mountains the stratified rocks dip W. at Whitehaven, N, at Hesket, E. at Shap, S. at Ulverston; from the chain of the Lammermuirs, they dip N. W. under the great valley of the Forth, and S. E. into Northumberland. The most general dip in England is easterly, the principal mountains being situated on the western border: from Brittany, the Ardennes, and Auvergne, the dips of the strata converge toward the low ground of the Basin of Paris; from the plains of Languedoc the strata rise toward the Pyrenees and the mountains of central France; the Pyrenees, the Apennines, the Alps, the Carpathians, the Grampians, are axes from which the stratified rocks decline, to pass under the lower ground on each side. Diag. No. 7.
It is generally found that the dip of the strata, thus obviously related in direction to the axes and centres of mountain groups, is also related to them in amount, so that the angular value of the dip—or the number of feet in one hundred that the strata decline—decreases ​continually from the mountains toward the plains, and in the middle of these is sometimes evanescent. Near London, for example, and on the coast of England generally, strata, though not level, dip moderately (1° or 2°) toward the east; but on the line to North Wales the dip augments; on the border of the Principality it measures 5°, 10°, 15°, and in the range of the Berwyn mountains, 30° and 40°, or still higher angles.
The direction of mountain chains, and the position of mountain groups, being extremely diversified, the lines of strike and dip of the strata which depend upon them are also very various. Perhaps in the progress of the science some law of these directions may be established: in the progress of this essay we shall examine one such attempt by a distinguished foreign geologist. At present the most important things taught us by the phenomena of dipping strata are these:—1. The dip is related to the elevation of ground; and 2. The strata do not descend from one mountain chain below the surface of plain countries more than a very moderate depth (four to five miles) before they begin to rise again toward another axis of elevated ground.
The principal mountain chains and groups are thus seen to be the axes of declination of the stratified rocks; and it was not without reason that De Saussure explored with so much patience the giant elevations of Switzerland, Dr. Hutton and Werner studied the Scottish and Saxon chains, and Mitchell with a grand generalisation referred to the leading features of physical geography as a basis of laws of geological phenomena. The axes of mountain chains and groups being before shown to be generally occupied by unstratified rocks, we have arrived at the important inference, that the dip, or deviation of stratified rocks from the horizontal position, depends on the same axes or centres as the exhibition of unstratified rocks: the production of the latter is therefore in some way connected with the declination of the former.
If we suppose the unstratified rocks to have been ​raised from below, the position of the strata, the relations of physical geography, and the relations of the two classes of rocks would be at once explained. In order to see what foundation may exist for such a speculation, let us inquire into further details and other cases of the position of stratified and unstratified rocks.
Local Declinations and unusual Positions of Strata, &c.
It is not only in mountainous regions that the strata are found dipping at high angles; the same phenomena are repeated on a smaller scale, and for smaller distances, at many points situated in the midst of the great basins of strata far from the principal axes of declination.
The appearances presented at these points of disturbed stratification are extremely various, but they admit of a simple and useful classification. Nothing is more common, in many large districts, than a slight elevation of the plane of stratification along a certain straight line, so that the rocks decline from it on both sides, as a, diag. No. 8. This is called an anticlinal axis, and the elevated ridge a saddle. Its converse (b), the line to which the strata decline, is called a synclinal axis, and the whole depression a trough.
It not unfrequently happens, on a small scale, as in the Craven district of Yorkshire, in the Abberley hills, Clee hills, the shores of Berwickshire, &c., and still more frequently and remarkably on a great scale, among the Alps (Vale of Chamouni, Lauterbrun, &c.), that the strata near an anticlinal axis, instead of being formed in evenly declining planes, are twisted and contorted in several directions, as if exceeding violence had been repeatedly exerted in lateral as well as vertical directions (c). In many instances (as on the line of the Penine fault near Crossfell, near Kirby Lonsdale, and near Lancaster), the strata are reared on end, so as to be nearly or actually vertical (d); in other rarer examples (Malvern hills) they are totally overthrown, or, after having been raised to a vertical position, the upper parts have been ​pushed outwards, so that the strata usually lying uppermost in the group are actually, for a short distance, undermost (e).
Faults.
Besides these, other forms of disturbed stratification demand attention; especially those in which the continuity of the strata is broken, and the divided parts placed at different levels. This interruption and dislocation of the strata commonly happens along a plane approaching to the vertical, which is usually marked by a rude and irregular fissure. This fissure, whether empty or in any manner filled (with fragments of the bordering rocks or other substances), is called "a fault," and locally "a dyke" "a trouble" "a gall" "a slip," &c.
The most simple and frequent case of faults is represented in the annexed vertical section (No. 9.) at the letter a, the strata lying nearly level, the fault vertical, the dislocation moderate in amount, and no particular bending of the rocks near it. In b the fault deviates
from the vertical by the angle x b y, and is said to have an underlay; the strata are considerably depressed, and in such a manner that a perpendicular dropped from b would fall clear of the edges of the depressed beds; not as in c, which represents a rare and exceptional case, so rare, indeed, that a clear example of it with a considerable depression of beds never occurred to the author, among very numerous instances studied in all classes ​of stratified rocks. In d the strata bend to the fault so as to coincide with its direction. In e the contrary effect is seen, the strata bending so as to meet the fault at right angles, as on the line of the great Tynedale fault, which disturbs the beds 1200 feet.
In some instances the fault fissures are open, as f, in others, full of angular dispersed fragments from the adjoining rocks (g); sometimes a leader of one or more of the softer strata follows up the fissure for a considerable distance (h); but frequently, as i, the fissure is closed. See the annexed vertical section.
The surfaces of the fissure accompanying a fault are often remarkable, and afford good evidence in favour of the dislocation of the masses having been accomplished by great mechanical violence, and perhaps a single continued effort. Let V fig. 11. be any vertical plane crossing f F F' F″, the plane of a fault fissure, which is accompanied by a dislocation of strata through the extent f F″; a b c, being the corresponding beds on the two sides of the fault. The face f F F' F″, one side of the fissure of the fault, is often scored by grooves (g g) parallel to the direction of the dislocation of the strata; that is to say, deep lines are ploughed on the broken ends of the rocks in the very direction in which they must have been produced, supposing, as the other phenomena indicate, the rocks to have slipped along the plane of the fault. A magnificent example of this is ​seen at Cullercoats in the great Tynedale, or 90 fathom dyke, of the Newcastle Coalfield.
Extent and Frequency of Faults.
The extent of vertical displacement occasioned by faults varies from a few inches or feet to thousands of feet; examples of the former are common, of the latter rare. When carefully studied, however, the principal difference between them the extent of the movement, is the only one which appears constant and essential. This is obviously related to the force employed in producing the fracture. That force may have been different in amount in the two cases contrasted, or different in the duration of its exertion; for the conformity of the circumstances of fracture seems to forbid a supposition of a different mode of action. Now, as an examination of the smaller and larger faults, when their planes can be clearly seen, appears to show that only one kind of action has been impressed upon the masses, as they appear to have slided in one direction, have been rubbed ​on their faces in one direction, and exhibit almost never any signs of repeated action along the same or neighbouring planes, we are forced to adopt, as a highly probable view of their origin, one continuous effort of a great force tending to extend, and, consequently inducing tension in, and fracture of, the crust of the globe. It appears no more necessary to suppose many interrupted efforts for a great fault like the Tynedale or Craven faults, of a thousand feet or yards, than for the numerous "hitches" in a colliery of one or a dozen feet.
It is commonly the case that such faults, when viewed on a horizontal plane, range nearly in straight lines, and for considerable, but very variable, lengths. When many faults occur, each producing only a moderate "throw," "shift," or displacement of the strata, their range is usually of only a few hundred yards, or a few miles, when they fall into and are stopped by some greater faults, or axes of movement.
On the contrary, when only a few faults occur in a district, and these have a great effect as to vertical movement, their course is usually of very considerable extent, even to many miles (the Tynedale and Craven faults range from 20 to 40 miles); but these also terminate in other faults or great centres or axes of movement. Faults which cross and appear to displace one another laterally, obey the same law of the angles as when their planes are compared to the surfaces of stratification, and the direction of vertical movements. (Fig. 9. b.) Faults are the most common of all the forms of disturbed stratification: but, except in particular cases, they are the least influential on the physical configuration of the country. All the rocks which are disturbed by any fault have experienced on one side the same movement, and to the same extent, excepting only those portions which have been subjected to violent pressure; and the bottom of the faults has never been reached, except when they terminate in another dislocation. ​
Relation of Faults, Mineral Veins, Dykes, &c., to the great Lines of disturbed Rocks.
It is noticed, as a circumstance of common occurrence, that mineral veins are no otherwise different from faults than by reason of the fissures which these have opened in the rocks being filled by sparry and metallic matters. This filling of a fissure constitutes a mineral vein; a similar fissure filled by basaltic or other rocks would be called a rock dyke; if occupied by clay and soft materials, a clay dyke. The point of importance in each of these cases is the mechanical formation of a fissure of the rocks along the plane of the fault; and it is to be determined by further inquiry, what was the cause of this particular line being followed by the disturbing force, and how the fissure, when made, came to be filled with its sparry, rocky, or soft argillaceous contents.
There appears to be some general relation observable between the lines of fault and the axes of great subterranean movement: that a "master fault" swallows up the smaller ones, or ramifies into them near the surface, has long been believed by the colliers of Somersetshire (as we learn from Dr. W. Smith). It appears, from our own and other researches, that the fissures accompanying mineral veins in the north of England, in the Penine Chain, and on the side of the Vale of Clwydd, terminate in such master faults; it also appears, by a careful analysis of phenomena, that mineral veins are so related to axes of disturbed strata, (like the Stiperstones, Greenhow hill, &c.), that they spring out from such, or tend to cross them at right angles, and scarcely appear anywhere abundant except in the vicinity of points or lines of great disruption of the rocks. Faults, dykes, and veins must, therefore, be referred, as to the origin of the fractures, to the same general cause which placed the strata of the mountains in their disturbed and inclined positions. ​
Before adopting, definitively, the conclusion obviously indicated by all the preceding facts, that the stratified rocks in the crust of the earth have been broken up, so that its disrupted masses have been placed in new positions, and that the unstratified rocks have been raised in consequence of such disruptions along the axes, and about the centres of mountain chains and groups, it will be proper to inquire further into the nature and origin of these two classes of rocks.
Origin of stratified and unstratified Rocks.
The great and leading distinction between these rocks, is the form of their whole masses; but, besides this, we observe, in other respects, very important differences, which facilitate investigations into their origin,—differences of internal structure, chemical character, mineral aggregation, and imbedded substances.
Stratification is a form of matter seldom produced in perfection among the effects of modern nature, except by the agency of water. The sediment from rivers, the deposits in lakes, the sandy and pebbly accumulations from the sea, all possess the true characters of stratification, for they tend to be produced in considerable breadths, with comparatively small thicknesses. And as among the ancient rocks we frequently find contiguous deposits of different chemical nature, as limestone succeeding clay or sandstone, so in these more modern products, similar successions of strata occur: clays and sands, and marly limestones of different colour, consistence, and chemical quality. Many of the ancient sandy strata are laminated parallel to the surface, as are the modern sediments from a river or the tide; others are irregularly composed of oblique laminæ, or ripple marked on the surface, as are the deposits from agitated rivers and tidal currents.
All the comparisons which can be made between ancient strata and analogous products of modern nature, ​appear clearly to evince their common origin, no other essential differences being discoverable between them, except the great thickness and extent of the ancient rocks; and could we raise for examination the bed of the Atlantic or the Mediterranean, perhaps a part of this discrepancy might vanish; for Donati's researches on the bed of the Adriatic show the great extent of the modern deposits in that sea.
The unstratified rocks tried by the same test, the form of their masses, can in no manner be paralleled to the productions of water. The dykes and veins which belong to the same class as the huge amorphous masses, and are often of the same kind of rock, do resemble in their forms, to a considerable degree, the known products of modern volcanoes: particular ancient unstratified rocks, as basalt, exist in forms, and under circumstances, very similar to analogous rocks, the fruit of volcanic fires.
The chemical composition of the two classes of rocks resembles in some points, and differs in others: they are in some points similar, for they contain some identical minerals, and many identical elementary substances; but numerous minerals are found in the unstratified rocks which are not known among the others. Limestone, sandstone, and clay, which constitute so many of the stratified masses, are forms of mineral aggregations such as never occur among granites, basalts, porphyries, &c., which make up a large portion of the unstratified rocks.
But the difference in their mineral aggregation is yet more remarkable. The ingredients of the stratified rocks appear almost always in such a state, as to suggest to the observer their aggregation from a state of solution, suspension, or drifting in water: limestone rocks, for instance, appear to have been collected, as smaller quantities are at this day, from the decomposition of water by chemical and vital agencies; clays were clearly collected from matter finely divided and diffused or suspended in water. Sandstones are as clearly the accumulation of grains of quartz, or other minerals worn and ​rounded in water, while conglomerates leave no more doubt of the former action of agitated water, than the pebbles of a river bed, or the sea shore.
On the contrary, the unstratified rocks are mostly crystallised; that is to say, their constituent ingredients are symmetrically arranged and bounded by regular surfaces, meeting at definite angles: they are not such as in general to be separately soluble in water at any temperature; they never show any marks of arrangement such as might arise from suspension or drifting, nor any such proofs of mechanical action, as worn grains of sand, or pebbles of rock. But their composition is in the great mass, and in the nature of the constituent crystals, always analogous, and frequently identical with, the known effects of heat in the furnace of the chemist, or the subterranean laboratory of nature.
Finally, these two classes of rocks differ essentially in another most important respect, which, taken in conjunction with the preceding facts, is quite decisive of their difference of origin: the stratified rocks are generally stored with the reliquiæ of plants and animals, even to a greater degree than modern marls, clays, and sands deposited from water; while the unstratified rocks contain none of these things, or if, by chance, a solitary shell has been found amongst such rocks, its inclusion is easily explained, just as by some accident volcanic scoriæ have been found to cover bones in Auvergne.
The animal remains found in the stratified rocks are chiefly marine, and nearly all aquatic; they occur, in many instances, under circumstances of position and relation which prove that they were often quietly buried or drifted by water from small distances, but sometimes worn to pebbles; just as from the deep and quiet sea we now dredge shells in complete preservation, their spines and ornaments perfect: while nearer the shore, worn shells, and under the cliffs, among the pebbles, rolled and fragmented particles appear.
It is, therefore, impossible to doubt that the ​stratified rocks, holding remains of aquatic animals or water drifted portions of land plants, were formed in water: this applies to the far greater number of the strata. But it is equally clear, that those strata which alternate with these, and do not yield organic remains, but are of the same general characters, and were, by marks of structure and aggregation, evidently produced in the same way, are also of watery origin. All the really stratified rocks, then, are the product of water; but the unstratified rocks are generally the fruit of the action of heat.
We must, therefore, here divide the subjects for consideration in the structure of the globe according to the aqueous or igneous agency concerned, and shall commence with the history of the deposits from water.
The most general view to which we are thus conducted, gives to all the stratified rocks an aqueous, and to the unstratified an igneous, origin: the former were deposited from above, in calm or agitated water, along the shores, in the depths of the sea, or in lakes; the latter were raised from below, by the excitement of internal heat. Subterranean movements affected the stratified rocks, and elevated them from their level position into mountain chains and ranges of hills, and the same influence, or an action consequent upon it, raised the fluid or solid unstratified rocks along the axes, or at the centres, of the elevator movements. Thus, it is a certain and general truth, that in the composition of the crust of the globe, in the arrangement of rocks in their present position, in the production of the physical features of our planet, both internal heat and the agency of external water have had their share; and by studying, carefully, the effects now produced, though apparently on a smaller scale, by the same natural agencies, under varied circumstances, we may hope to arrive at correct general inferences as to the manner in which even the grandest and most surprising of the old revolutions of nature were occasioned. ​
Relative Periods of disturbed Stratification.
One of the most remarkable of all the results yet arrived at, by combining the study of the two classes of rocks just distinguished, is the certainty that the subterranean movements of the solid crust of the globe, to which the deranged positions of the strata are owing, were not all of the same date; but that some mountain ridges, and some lines and points of unstratified rocks, had been uplifted before others,—some strata disturbed, before others were formed. The mode by which this has been ascertained is extremely simple. When, as in the section (fig. 12.), certain old strata (a, b, c, &c.) are found displaced
from their original nearly level position, and thrown to high angles of elevation, and other more recent strata (h i k) are placed level against the slopes, or even covering the ends of the former, it is plain that the dislocation of a b c happened in the interval of geological time which occurred between the completion of the newest bed (f) of the dislocated, and the oldest bed (h) of the undisturbed deposits. On different chains of mountains, along different lines of faults, &c., the period when the disturbances happened, judged of by this test, is found to be often very different. One of the most singular examples of this dislocation of some strata, in districts where others of more recent deposition remain undisturbed, was noticed by Dr. William Smith, in 1791, at Pucklechurch in Gloucestershire, and in Somersetshire. The coal formation is here found dipping at a high ​angle below, and covered up by horizontal strata of red marl and lias, thus:
In Yorkshire and Durham the same thing is observed with respect to the magnesian limestone and the coal, with the addition that the coal strata are broken by faults, which do not affect the limestone above; thus:
The principal difficulty in applying this very simple mode of determination to particular cases, so as to class all faults and other effects of subterranean movements according to the date of their occurrence, consists in ascertaining the indispensable data of what strata are and what are not disturbed along a given line, or at a certain point. When the undisturbed strata lie upon, or abut clearly against, or plainly surround the disturbed rocks, the evidence is satisfactory, and easily verified; but, in most cases, this clear testimony is wanting, and it is by considering the relative directions and relative dips of the two sets of strata (the disturbed and undisturbed), that we are to arrive at a determination of the question. The following notices and sketches will illustrate this point. ​
Whenever, in any district, the stratified rocks, instead of all lying parallel to one another, suffering the same deviations from horizontality, bending in the same flexures, dropping or rising by the same faults, and regarding in their declinations the same axis or centre of dips, divide themselves, in these respects, into two or more sets, which differ from one another in all or any of these respects, there is said to be unconformity of the strata. The place of this unconformity is the interval between the two sets thus disagreeing; it is said to occur between the oldest of one and the newest of the other set; it affects the geographical distribution of the strata, as shown on a map, and their relative inclination and exposures, as shown in a section. Thus in the map diagram (fig. 15.),
the series of strata marked a b c d are parallel in one set, and e f g in another; but their directions, or strikes, (S, S and s s) on the surface differ, and the lowest of the upper set (e) rests in one place on a, in another on b; in another on c or d.; the dips, D and d, are in different directions.
In a section (as fig. 16.), some difference of inclination ​commonly occurs. But when the strikes and dips of either the upper or lower set vary, which is a very common case, the same district, as Yorkshire or Derbyshire, may exhibit in the same region both conformity and unconformity between the same two sets of beds, as in the
map diagram (fig. 17); where a b c d are coal strata, with variable dips and strikes, generally unconfirmed, but on the line W E, for a short distance, conformed to the magnesian strata lying upon them (e f g h). Yet, in this case, the section on the line W E would exhibit a want of conformity in the dip (as in fig. 16.), the beds a b c d being more inclined than e f g h. When the strata are not in contact, or, for any reason the junction cannot be clearly seen, many observations of dip and strike in each set of beds will in general determine the existence of unconformity: but it would be folly to rest so important a decision upon testimony less demonstrative than the country will yield; and, in some cases, sufficient evidence is unattainable by any exertion of industry and skill.