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diff --git a/old/3772-h/files/ch5.html b/old/3772-h/files/ch5.html new file mode 100644 index 0000000..3be4037 --- /dev/null +++ b/old/3772-h/files/ch5.html @@ -0,0 +1,1233 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Lyell/The Student's Elements of Geology --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Student's Elements of Geology: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv= +"Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="geology.css" type="text/css"> +</head> +<body> +<p><b>The Student’s Elements of Geology</b></p> + +<hr> +<p class="page"><a name="page 70">[ 70 ]</a></p> + +<p> </p> + +<center><b>Chapter V</b><br> +<br> +ELEVATION OF STRATA ABOVE THE SEA.—HORIZONTAL AND INCLINED +STRATIFICATION.</center> + +<p class="intro">Why the Position of Marine Strata, above the Level +of the Sea, should be referred to the rising up of the Land, not to +the going down of the Sea. — Strata of Deep-sea and +Shallow-water Origin alternate. — Also Marine and Fresh-water +Beds and old Land Surfaces. — Vertical, inclined, and folded +Strata. — Anticlinal and Synclinal Curves. — Theories +to explain Lateral Movements. — Creeps in Coal-mines. — +Dip and Strike. — Structure of the Jura. — Various +Forms of Outcrop. — Synclinal Strata forming Ridges. — +Connection of Fracture and Flexure of Rocks. — Inverted +Strata. — Faults described. — Superficial Signs of the +same obliterated by Denudation. — Great Faults the Result of +repeated Movements. — Arrangement and Direction of parallel +Folds of Strata. — Unconformability. — Overlapping +Strata.</p> + +<p><b>Land has been raised, not the Sea +lowered.</b>—It has been already stated that the +aqueous rocks containing marine fossils extend over wide +continental tracts, and are seen in mountain chains rising to great +heights above the level of the sea <a href="ch1.html#page 29">(p. +29)</a>. Hence it follows, that what is now dry land was once under +water. But if we admit this conclusion, we must imagine, either +that there has been a general lowering of the waters of the ocean, +or that the solid rocks, once covered by water, have been raised up +bodily out of the sea, and have thus become dry land. The earlier +geologists, finding themselves reduced to this alternative, +embraced the former opinion, assuming that the ocean was originally +universal, and had gradually sunk down to its actual level, so that +the present islands and continents were left dry. It seemed to them +far easier to conceive that the water had gone down, than that +solid land had risen upward into its present position. It was, +however, impossible to invent any satisfactory hypothesis to +explain the disappearance of so enormous a body of water throughout +the globe, it being necessary to infer that the ocean had once +stood at whatever height marine shells might be detected. It +moreover appeared clear, as the science of geology advanced, that +certain spaces on the globe had been alternately sea, then land, +then estuary, then sea again, and, lastly, once more habitable +land, having remained in each of these states for considerable +periods. In order to account for such phenomena</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 71">[ 71 ]</a></p> + +<p>without admitting any movement of the land itself, we are +required to imagine several retreats and returns of the ocean; and +even then our theory applies merely to cases where the marine +strata composing the dry land are horizontal, leaving unexplained +those more common instances where strata are inclined, curved, or +placed on their edges, and evidently not in the position in which +they were first deposited.</p> + +<p>Geologists, therefore, were at last compelled to have recourse +to the doctrine that the solid land has been repeatedly moved +upward or downward, so as permanently to change its position +relatively to the sea. There are several distinct grounds for +preferring this conclusion. First, it will account equally for the +position of those elevated masses of marine origin in which the +stratification remains horizontal, and for those in which the +strata are disturbed, broken, inclined, or vertical. Secondly, it +is consistent with human experience that land should rise gradually +in some places and be depressed in others. Such changes have +actually occurred in our own days, and are now in progress, having +been accompanied in some cases by violent convulsions, while in +others they have proceeded so insensibly as to have been +ascertainable only by the most careful scientific observations, +made at considerable intervals of time. On the other hand, there is +no evidence from human experience of a rising or lowering of the +sea’s level in any region, and the ocean can not be raised or +depressed in one place without its level being changed all over the +globe.</p> + +<p>These preliminary remarks will prepare the reader to understand +the great theoretical interest attached to all facts connected with +the position of strata, whether horizontal or inclined, curved or +vertical.</p> + +<p>Now the first and most simple appearance is where strata of +marine origin occur above the level of the sea in horizontal +position. Such are the strata which we meet with in the south of +Sicily, filled with shells for the most part of the same species as +those now living in the Mediterranean. Some of these rocks rise to +the height of more than 2000 feet above the sea. Other mountain +masses might be mentioned, composed of horizontal strata of high +antiquity, which contain fossil remains of animals wholly +dissimilar from any now known to exist. In the south of Sweden, for +example, near Lake Wener, the beds of some of the oldest +fossiliferous deposits, called Silurian and Cambrian by geologists, +occur in as level a position as if they had recently formed part of +the delta of a great river, and been left dry</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 72">[ 72 ]</a></p> + +<p>on the retiring of the annual floods. Aqueous rocks of equal +antiquity extend for hundreds of miles over the lake-district of +North America, and exhibit in like manner a stratification nearly +undisturbed. The Table Mountain at the Cape of Good Hope is another +example of highly elevated yet perfectly horizontal strata, no less +than 3500 feet in thickness, and consisting of sandstone of very +ancient date.</p> + +<p>Instead of imagining that such fossiliferous rocks were always +at their present level, and that the sea was once high enough to +cover them, we suppose them to have constituted the ancient bed of +the ocean, and to have been afterwards uplifted to their present +height. This idea, however startling it may at first appear, is +quite in accordance, as before stated, with the analogy of changes +now going on in certain regions of the globe. Thus, in parts of +Sweden, and the shores and islands of the Gulf of Bothnia, proofs +have been obtained that the land is experiencing, and has +experienced for centuries, a slow upheaving movement.*</p> + +<p>It appears from the observations of Mr. Darwin and others, that +very extensive regions of the continent of South America have been +undergoing slow and gradual upheaval, by which the level plains of +Patagonia, covered with recent marine shells, and the Pampas of +Buenos Ayres, have been raised above the level of the sea. On the +other hand, the gradual sinking of the west coast of Greenland, for +the space of more than 600 miles from north to south, during the +last four centuries, has been established by the observations of a +Danish naturalist, Dr. Pingel. And while these proofs of +continental elevation and subsidence, by slow and insensible +movements, have been recently brought to light, the evidence has +been daily strengthened of continued changes of level effected by +violent convulsions in countries where earthquakes are frequent. +There the rocks are rent from time to time, and heaved up or thrown +down several feet at once, and disturbed in such a manner as to +show how entirely the original position of strata may be modified +in the course of centuries.</p> + +<p>Mr. Darwin has also inferred that, in those seas where circular +coral islands and barrier reefs abound, there is a slow and +continued sinking of the submarine mountains on which the masses of +coral are based; while there are other areas of the South Sea where +the land is on the rise, and where coral has been upheaved far +above the sea-level.</p> + +<p><b>Alternations of Marine and Fresh-water +Strata.</b>—It has been shown in the third chapter +that there is such a difference</p> + +<p class="fnote">* See "Principles of Geology," 1867, p. 314.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 73">[ 73 ]</a></p> + +<p>between land, fresh-water, and marine fossils as to enable the +geologist to determine whether particular groups of strata were +formed at the bottom of the ocean or in estuaries, rivers, or +lakes. If surprise was at first created by the discovery of marine +corals and shells at the height of several miles above the +sea-level, the imagination was afterwards not less startled by +observing that in the successive strata composing the earth’s +crust, especially if their total thickness amounted to thousands of +feet, they comprised in some parts formations of shallow-sea as +well as of deep-sea origin; also beds of brackish or even of purely +fresh-water formation, as well as vegetable matter or coal +accumulated on ancient land. In these cases we as frequently find +fresh-water beds below a marine set or shallow-water under those of +deep-sea origin as the reverse. Thus, if we bore an artesian well +below London, we pass through a marine clay, and there reach, at +the depth of several hundred feet, a shallow-water and fluviatile +sand, beneath which comes the white chalk originally formed in a +deep sea. Or if we bore vertically through the chalk of the North +Downs, we come, after traversing marine chalky strata, upon a +fresh-water formation many hundreds of feet thick, called the +Wealden, such as is seen in Kent and Surrey, which is known in its +turn to rest on purely marine beds. In like manner, in various +parts of Great Britain we sink vertical shafts through marine +deposits of great thickness, and come upon coal which was formed by +the growth of plants on an ancient land-surface sometimes hundreds +of square miles in extent.</p> + +<p><b>Vertical, Inclined, and Curved +Strata.</b>—It has been stated that marine strata of +different ages are sometimes found at a considerable height above +the sea, yet retaining their original horizontality; but this state +of things is quite exceptional. As a general rule, strata are +inclined or bent in such a manner as to imply that their original +position has been altered.</p> + +<img src="../images/fig54.jpg" width="172" height="152" alt= +"Fig. 54: Vertical conglomerate and sandstone." align="right"> + +<p>The most unequivocal evidence of such a change is afforded by +their standing up vertically, showing their edges, which is by no +means a rare phenomenon, especially in mountainous countries. Thus +we find in Scotland, on the southern skirts of the Grampians, beds +of pudding-stone alternating with thin layers of fine sand, all +placed vertically to the horizon. When Saussure first observed +certain conglomerates in a</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 74">[ 74 ]</a></p> + +<p>similar position in the Swiss Alps, he remarked that the +pebbles, being for the most part of an oval shape, had their longer +axes parallel to the planes of stratification (see Fig. 54 on +preceding page). From this he inferred that such strata must, at +first, have been horizontal, each oval pebble having settled at the +bottom of the water, with its flatter side parallel to the horizon, +for the same reason that an egg will not stand on either end if +unsupported. Some few, indeed, of the rounded stones in a +conglomerate occasionally afford an exception to the above rule, +for the same reason that in a river’s bed, or on a shingle beach, +some pebbles rest on their ends or edges; these having been shoved +against or between other stones by a wave or current, so as to +assume this position.</p> + +<p><b>Anticlinal and Synclinal +Curves.</b>—Vertical strata, when they can be traced +continuously upward or downward for some depth, are almost +invariably seen to be parts of great curves, which may have a +diameter of a few yards, or of several miles. I shall first +describe two curves of considerable regularity, which occur in +Forfarshire, extending over a country twenty miles in breadth, from +the foot of the Grampians to the sea near Arbroath.</p> + +<center><img src="../images/fig55.jpg" width="597" height="197" alt= +"Fig. 55: Section of Forfarshire, from N.W. to S.E."></center> + +<p>The mass of strata here shown may be 2000 feet in thickness, +consisting of red and white sandstone, and various coloured shales, +the beds being distinguishable into four principal groups, namely, +No. 1, red marl or shale; No. 2, red sandstone, used for building; +No. 3, conglomerate; and No. 4, grey paving-stone, and tile-stone, +with green and reddish shale, containing peculiar organic remains. +A glance at the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 75">[ 75 ]</a></p> + +<p>section will show that each of the formations 2, 3, 4 are +repeated thrice at the surface, twice with a southerly, and once +with a northerly inclination or <i>dip</i>, and the beds in No. 1, +which are nearly horizontal, are still brought up twice by a slight +curvature to the surface, once on each side of A. Beginning at the +north-west extremity, the tile-stones and conglomerates, No. 4 and +No. 3, are vertical, and they generally form a ridge parallel to +the southern skirts of the Grampians. The superior strata, Nos. 2 +and 1, become less and less inclined on descending to the valley of +Strathmore, where the strata, having a concave bend, are said by +geologists to lie in a “ trough" or “ basin." Through the centre of +this valley runs an imaginary line A, called technically a +“ synclinal line," where the beds, which are tilted in opposite +directions, may be supposed to meet. It is most important for the +observer to mark such lines, for he will perceive by the diagram +that, in travelling from the north to the centre of the basin, he +is always passing from older to newer beds; whereas, after crossing +the line A, and pursuing his course in the same southerly +direction, he is continually leaving the newer, and advancing upon +older strata. All the deposits which he had before examined begin +then to recur in reversed order, until he arrives at the central +axis of the Sidlaw hills, where the strata are seen to form an +arch, or <i>saddle</i>, having an <i>anticlinal</i> line, B, in the +centre. On passing this line, and continuing towards the S.E., the +formations 4, 3, and 2, are again repeated, in the same relative +order of superposition, but with a southerly dip. At Whiteness (see +Fig. 55) it will be seen that the inclined strata are covered by a +newer deposit, <i>a</i>, in horizontal beds. These are composed of +red conglomerate and sand, and are newer than any of the groups, 1, +2, 3, 4, before described, and rest <i>unconformably</i> upon +strata of the sandstone group, No. 2.</p> + +<p>An example of curved strata, in which the bends or convolutions +of the rock are sharper and far more numerous within an equal +space, has been well described by Sir James Hall.* It occurs near +St. Abb’s Head, on the east coast of Scotland, where the rocks +consist principally of a bluish slate, having frequently a +ripple-marked surface. The undulations of the beds reach from the +top to the bottom of cliffs from 200 to 300 feet in height, and +there are sixteen distinct bendings in the course of about six +miles, the curvatures being alternately concave and convex +upward.</p> + +<p><b>Folding by Lateral +Movement.</b>—An experiment was made by Sir James +Hall, with a view of illustrating the manner in</p> + +<p class="fnote">* Edin. Trans., vol. vii, pl. 3.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 76">[ 76 ]</a></p> + +<center><img src="../images/fig56.jpg" width="347" height="221" alt= +"Fig. 56: Curved strata of slate near St. Abb's Head, Berwickshire."> +</center> + +<p>which such strata, assuming them to have been originally +horizontal, may have been forced into their present position. A set +of layers of clay were placed under a weight, and their opposite +ends pressed towards each other with such force as to cause them to +approach more nearly together. On the removal of the weight, the +layers of clay were found to be curved and folded, so as to bear a +miniature resemblance to the strata in the cliffs. We must, +however, bear in mind that in the natural section or sea-cliff we +only see the foldings imperfectly, one part being invisible beneath +the sea, and the other, or upper portion, being supposed to have +been carried away by <i>denudation</i>, or that action of water +which will be explained in the next chapter. The dark lines in the +plan (Fig. 57) represent what is actually seen of the strata in +the line of cliff alluded to; the fainter lines, that portion which +is concealed beneath the sea-level, as also that which is supposed +to have once existed above the present surface.</p> + +<center><img src="../images/fig57.jpg" width="345" height="195" alt= +"Fig. 57"></center> + +<p> </p> + +<hr> +<p class="page"><a name="page 77">[ 77 ]</a></p> + +<p>We may still more easily illustrate the effects which a lateral +thrust might produce on flexible strata, by placing several pieces +of differently coloured cloths upon a table, and when they are +spread out horizontally, cover them with a book. Then apply other +books to each end, and force them towards each other. The folding +of the cloths (see Fig. 58) will imitate those of the bent strata; +the incumbent book being slightly lifted up, and no longer touching +the two volumes on which it rested before, because it is supported +by the tops of the anticlinal ridges formed by the curved cloths. +In like manner there can be no doubt that the squeezed strata, +although laterally condensed and more closely packed, are yet +elongated and made to rise upward, in a direction perpendicular to +the pressure.</p> + +<center><img src="../images/fig58.jpg" width="358" height="205" alt= +"Fig. 58"></center> + +<p>Whether the analogous flexures in stratified rocks have really +been due to similar sideway movements is a question which we can +not decide by reference to our own observation. Our inability to +explain the nature of the process is, perhaps, not simply owing to +the inaccessibility of the subterranean regions where the +mechanical force is exerted, but to the extreme slowness of the +movement. The changes may sometimes be due to variation in the +temperature of mountain masses of rock causing them, while still +solid, to expand or contract; or melting them, and then again +cooling them and allowing them to crystallise. If such be the case, +we have scarcely more reason to expect to witness the operation of +the process within the limited periods of our scientific +observation than to see the swelling of the roots of a tree, by +which, in the course of years, a wall of solid masonry may be +lifted up, rent or thrown down. In both instances the force may be +irresistible, but though adequate, it need not be visible by us, +provided the time required for its development be very great. The +lateral pressure arising from the unequal expansion of rocks by +heat may cause one mass lying in the same horizontal plane +gradually to occupy</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 78">[ 78 ]</a></p> + +<p>a larger space, so as to press upon another rock, which, if +flexible, may be squeezed into a bent and folded form. It will also +appear, when the volcanic and granitic rocks are described, that +some of them have, when melted in the interior of the earth’s +crust, been injected forcibly into fissures, and after the +solidification of such intruded matter, other sets of rents, +crossing the first, have been formed and in their turn filled by +melted rock. Such repeated injections imply a stretching, and often +upheaval, of the whole mass.</p> + +<p>We also know, especially by the study of regions liable to +earthquakes, that there are causes at work in the interior of the +earth capable of producing a sinking in of the ground, sometimes +very local, but often extending over a wide area. The continuance +of such a downward movement, especially if partial and confined to +linear areas, may produce regular folds in the strata.</p> + +<p><b>Creeps in Coal-mines.</b>—The +“creeps,” as they are called in coal-mines, afford an excellent +illustration of this fact.--First, it may be stated generally, that +the excavation of coal at a considerable depth causes the mass of +overlying strata to sink down bodily, even when props are left to +support the roof of the mine. “In Yorkshire,” says Mr. Buddle, +“three distinct subsidences were perceptible at the surface, after +the clearing out of three seams of coal below, and innumerable +vertical cracks were caused in the incumbent mass of sandstone and +shale which thus settled down.”* The exact amount of depression in +these cases can only be accurately measured where water accumulates +on the surface, or a railway traverses a coal-field.</p> + +<p>When a bed of coal is worked out, pillars or rectangular masses +of coal are left at intervals as props to support the roof, and +protect the colliers. Thus in Fig. 59, representing a section at +Wallsend, Newcastle, the galleries which have been excavated are +represented by the white spaces <i>a, b,</i> while the adjoining +dark portions are parts of the original coal seam left as props, +beds of sandy clay or shale constituting the floor of the mine. +When the props have been reduced in size, they are pressed down by +the weight of overlying rocks (no less than 630 feet thick) upon +the shale below, which is thereby squeezed and forced up into the +open spaces.</p> + +<p>Now it might have been expected that, instead of the floor +rising up, the ceiling would sink down, and this effect, called a +“thrust,” does, in fact, take place where the pavement is more +solid than the roof. But it usually happens, in coal-</p> + +<p class="fnote">* Proceedings of Geol. Soc., vol. iii, p. 148.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 79">[ 79 ]</a></p> + +<p>mines, that the roof is composed of hard shale, or occasionally +of sandstone, more unyielding than the foundation, which often +consists of clay. Even where the argillaceous substrata are hard at +first, they soon become softened and reduced to a plastic state +when exposed to the contact of air and water in the floor of a +mine.</p> + +<center><img src="../images/fig59.jpg" width="390" height="266" alt= +"Fig. 59: Section of carboniferous strata at Wallsend showing 'creeps'."> +</center> + +<p>The first symptom of a “creep,” says Mr. Buddle, is a slight +curvature at the bottom of each gallery, as at <i>a</i>, Fig. 59: +then the pavement, continuing to rise, begins to open with a +longitudinal crack, as at <i>b</i>; then the points of the +fractured ridge reach the roof, as at <i>c</i>; and, lastly, the +upraised beds close up the whole gallery, and the broken portions +of the ridge are reunited and flattened at the top, exhibiting the +flexure seen at <i>d.</i> Meanwhile the coal in the props has +become crushed and cracked by pressure. It is also found that below +the creeps <i>a, b, c, d,</i> an inferior stratum, called the +“metal coal,” which is 3 feet thick, has been fractured at the +points <i>e, f, g, h,</i> and has risen, so as to prove that the +upward movement, caused by the working out of the “main coal,” has +been propagated through a thickness of 54 feet of argillaceous +beds, which intervene between the two coal-seams. This same +displacement has also been traced downward more than 150 feet below +the metal coal, but it grows continually less and less until it +becomes imperceptible.</p> + +<p>No part of the process above described is more deserving of our +notice than the slowness with which the change in the arrangement +of the beds is brought about. Days,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 80">[ 80 ]</a></p> + +<p>months, or even years, will sometimes elapse between the first +bending of the pavement and the time of its reaching the roof. +Where the movement has been most rapid, the curvature of the beds +is most regular, and the reunion of the fractured ends most +complete; whereas the signs of displacement or violence are +greatest in those creeps which have required months or years for +their entire accomplishment. Hence we may conclude that similar +changes may have been wrought on a larger scale in the earth’s +crust by partial and gradual subsidences, especially where the +ground has been undermined throughout long periods of time; and we +must be on our guard against inferring sudden violence, simply +because the distortion of the beds is excessive.</p> + +<p>Engineers are familiar with the fact that when they raise the +level of a railway by heaping stone or gravel on a foundation of +marsh, quicksand, or other yielding formation, the new mound often +sinks for a time as fast as they attempt to elevate it; when they +have persevered so as to overcome this difficulty, they frequently +find that some of the adjoining flexible ground has risen up in one +or more parallel arches or folds, showing that the vertical +pressure of the sinking materials has given rise to a lateral +folding movement.</p> + +<p>In like manner, in the interior of the earth, the solid parts of +the earth’s crust may sometimes, as before mentioned, be made to +expand by heat, or may be pressed by the force of steam against +flexible strata loaded with a great weight of incumbent rocks. In +this case the yielding mass, squeezed, but unable to overcome the +resistance which it meets with in a vertical direction, may be +gradually relieved by lateral folding.</p> + +<img src="../images/fig60.jpg" width="191" height="99" alt="Fig. 60" +align="left"> + +<p><b>Dip and Strike.</b>—In +describing the manner in which strata depart from their original +horizontality, some technical terms, such as “dip” and “strike,” +“anticlinal” and “synclinal” line or axis, are used by geologists. +I shall now proceed to explain some of these to the student. If a +stratum or bed of rock, instead of being quite level, be inclined +to one side, it is said to <i>dip</i>; the point of the compass to +which it is inclined is called the <i>point of dip</i>, and the +degree of deviation from a level or horizontal line is called <i> +the amount of dip</i>, or <i>the angle of dip.</i> Thus, in the +annexed diagram (Fig. 60), a series of strata are inclined, and +they dip to the north at an angle of forty-five</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 81">[ 81 ]</a></p> + +<p>degrees. The <i>strike</i>, or <i>line of bearing</i>, is the +prolongation or extension of the strata in a direction <i>at right +angles</i> to the dip; and hence it is sometimes called the <i> +direction</i> of the strata. Thus, in the above instance of strata +dipping to the north, their strike must necessarily be east and +west. We have borrowed the word from the German geologists, <i> +streichen</i> signifying to extend, to have a certain direction. +Dip and strike may be aptly illustrated by a row of houses running +east and west, the long ridge of the roof representing the strike +of the stratum of slates, which dip on one side to the north, and +on the other to the south.</p> + +<p>A stratum which is horizontal, or quite level in all directions, +has neither dip nor strike.</p> + +<p>It is always important for the geologist, who is endeavouring to +comprehend the structure of a country, to learn how the beds dip in +every part of the district; but it requires some practice to avoid +being occasionally deceived, both as to the point of dip and the +amount of it.</p> + +<center><img src="../images/fig61.jpg" width="333" height="192" alt= +"Fig. 61: Apparent horizontality of inclined strata."></center> + +<p>If the upper surface of a hard stony stratum be uncovered, +whether artificially in a quarry, or by waves at the foot of a +cliff, it is easy to determine towards what point of the compass +the slope is steepest, or in what direction water would flow if +poured upon it. This is the true dip. But the edges of highly +inclined strata may give rise to perfectly horizontal lines in the +face of a vertical cliff, if the observer see the strata in the +line of the strike, the dip being inward from the face of the +cliff. If, however, we come to a break in the cliff, which exhibits +a section exactly at right angles to the line of the strike, we are +then able to ascertain the true dip. In the drawing (Fig. 61), we +may suppose a headland, one side of which faces to the north, where +the beds would appear perfectly horizontal to a person in the boat; +while in the other side facing the west, the true dip</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 82">[ 82 ]</a></p> + +<p>would be seen by the person on shore to be at an angle of +40°. If, therefore, our observations are confined to a vertical +precipice facing in one direction, we must endeavour to find a +ledge or portion of the plane of one of the beds projecting beyond +the others, in order to ascertain the true dip.</p> + +<img src="../images/fig62.jpg" width="174" height="188" alt= +"Fig. 62: Two hands used to determine the inclination of strata." +align="left"> + +<p>If not provided with a clinometer, a most useful instrument, +when it is of consequence to determine with precision the +inclination of the strata, the observer may measure the angle +within a few degrees by standing exactly opposite to a cliff where +the true dip is exhibited, holding the hands immediately before the +eyes, and placing the fingers of one in a perpendicular, and of the +other in a horizontal position, as in Fig. 62. It is thus easy to +discover whether the lines of the inclined beds bisect the angle of +90°, formed by the meeting of the hands, so as to give an angle +of 45°, or whether it would divide the space into two equal or +unequal portions. You have only to change hands to get the line of +dip on the upper side of the horizontal hand.</p> + +<center><img src="../images/fig63.jpg" width="334" height="179" alt= +"Fig. 63: Section illustrating the structure of the Swiss Jura."> +</center> + +<p>It has been already seen, in describing the curved strata on the +east coast of Scotland, in Forfarshire and Berwickshire, that a +series of concave and convex bendings are occasionally repeated +several times. These usually form part of a series of parallel +waves of strata, which are prolonged in the same direction, +throughout a considerable extent of country. Thus, for example, in +the Swiss Jura, that lofty chain of mountains has been proved to +consist of many parallel ridges, with intervening longitudinal +valleys, as in Fig. 63, the ridges being formed by curved +fossiliferous strata,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 83">[ 83 ]</a></p> + +<p>of which the nature and dip are occasionally displayed in deep +transverse gorges, called “cluses,” caused by fractures at right +angles to the direction of the chain.* Now let us suppose these +ridges and parallel valleys to run north and south, we should then +say that the <i>strike</i> of the beds is north and south, and the +<i>dip</i> east and west. Lines drawn along the summits of the +ridges, A, B, would be anticlinal lines, and one following the +bottom of the adjoining valleys a synclinal line.</p> + +<p><img src="../images/fig64.jpg" width="225" height="144" alt= +"Fig. 64: Ground-plan of the denuded ridge C, Fig. 63. Fig. 65: Transverse section." align="right"></p> + +<p><b>Outcrop of Strata.</b>—It +will be observed that some of these ridges, A, B, are unbroken on +the summit, whereas one of them, C, has been fractured along the +line of strike, and a portion of it carried away by denudation, so +that the ridges of the beds in the formations <i>a, b, c</i> come +out to the day, or, as the miners say, <i>crop out</i>, on the +sides of a valley. The ground-plan of such a denuded ridge as C, as +given in a geological map, may be expressed by the diagram, Fig. +64, and the cross-section of the same by Fig. 65. The line D E, +Fig. 64, is the anticlinal line, on each side of which the dip is +in opposite directions, as expressed by the arrows. The emergence +of strata at the surface is called by miners their <i>outcrop</i>, +or <i>basset.</i></p> + +<p>If, instead of being folded into parallel ridges, the beds form +a boss or dome-shaped protuberance, and if we suppose the summit of +the dome carried off, the ground-plan would exhibit the edges of +the strata forming a succession of circles, or ellipses, round a +common centre. These circles are the lines of strike, and the dip +being always at right angles is inclined in the course of the +circuit to every point of the compass, constituting what is termed +a quâ-quâversal dip--that is, turning every way.</p> + +<p>There are endless variations in the figures described by the +basset-edges of the strata, according to the different inclination +of the beds, and the mode in which they happen to have been +denuded. One of the simplest rules, with which every geologist +should be acquainted, relates to the V-like form of the beds as +they crop out in an ordinary valley. First, if the strata be +horizontal, the V-like form will be also on a level, and the newest +strata will appear at the greatest heights.</p> + +<p class="fnote">* Thurmann, “Essai sur les Soulèvemens +Jurassiques de Porrentruy,” Paris, 1832.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 84">[ 84 ]</a></p> + +<img src="../images/fig66.jpg" width="263" height="614" alt= +"Fig. 66: Slope of valley 40°, dip of strata 20°. Fig. 67: Slope of valley 20°, dip of strata 50°. Fig. 68: Slope of valley 20°, dip of strata 20°, in opposite directions." + align="left"> + +<p>Secondly, if the beds be inclined and intersected by a valley +sloping in the same direction, and the dip of the beds be less +steep than the slope of the valley, then the V’s, as they are often +termed by miners, will point upward (see Fig. 66), those formed by +the newer beds appearing in a superior position, and extending +highest up the valley, as A is seen above B.</p> + +<p>Thirdly, if the dip of the beds be steeper than the slope of the +valley, then the V’s will point downward (see Fig. 67), and those +formed of the older beds will now appear uppermost, as B appears +above A.</p> + +<p>Fourthly, in every case where the strata dip in a contrary +direction to the slope of the valley, whatever be the angle of +inclination, the newer beds will appear the highest, as in the +first and second cases. This is shown by the drawing (Fig. 68), +which exhibits strata rising at an angle of 20°, and crossed by +a valley, which declines in an opposite direction at 20°.</p> + +<p>These rules may often be of great practical utility; for the +different degrees of dip occurring in the two cases represented +in</p> + +<p> </p> + +<p> </p> + +<hr> +<p class="page"><a name="page 85">[ 85 ]</a></p> + +<p>Figs. 66 and 67 may occasionally be encountered in following the +same line of flexure at points a few miles distant from each other. +A miner unacquainted with the rule, who had first explored the +valley Fig. 66, may have sunk a vertical shaft below the coal-seam +A, until he reached the inferior bed, B. He might then pass to the +valley, Fig. 67, and discovering there also the outcrop of two +coal-seams, might begin his workings in the uppermost in the +expectation of coming down to the other bed A, which would be +observed cropping out lower down the valley. But a glance at the +section will demonstrate the futility of such hopes.*</p> + +<center><img src="../images/fig69.jpg" width="385" height="122" alt= +"Fig. 69: Section of carboniferous rocks of Lancashire."></center> + +<p><b>Synclinal Strata forming +Ridges.</b>—Although in many cases an anticlinal axis +forms a ridge, and a synclinal axis a valley, as in A B, Fig. 63, +yet this can by no means be laid down as a general rule, as the +beds very often slope inward from either side of a mountain, as at +<i>a, b,</i> Fig. 69, while in the intervening valley, <i>c</i>, +they slope upward, forming an arch.</p> + +<p>It would be natural to expect the fracture of solid rocks to +take place chiefly where the bending of the strata has been +sharpest, and such rending may produce ravines giving access to +running water and exposing the surface to atmospheric waste. The +entire absence, however, of such cracks at points where the strain +must have been greatest, as at <i>a</i>, Fig. 63, is often very +remarkable, and not always easy of explanation. We must imagine +that many strata of limestone, chert, and other rocks which are now +brittle, were pliant when bent into their present position. They +may have owed their flexibility in part to the fluid matter which +they contained in their minute pores, as before described <a href= +"ch4.html#page 62">p. 62</a> and in part to the permeation of +sea-water while they were yet submerged.</p> + +<p class="fnote">* I am indebted to the kindness of T. Sopwith, +Esq., for three models which I have copied in the above diagrams; +but the beginner may find it by no means easy to understand such +copies, although, if he were to examine and handle the originals, +turning them about in different ways, he would at once comprehend +their meaning, as well as the import of others far more +complicated, which the same engineer has constructed to illustrate +<i>faults.</i><br> +† Edward Hull, Quart. Geol. Journ., vol. xxiv, p. 324, +1868.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 86">[ 86 ]</a></p> + +<center><img src="../images/fig70.jpg" width="319" height="100" alt= +"Fig. 70: Strata of chert, grit, and marl, near St. Jean de Luz."> +</center> + +<p>At the western extremity of the Pyrenees, great curvatures of +the strata are seen in the sea-cliffs, where the rocks consist of +marl, grit, and chert. At certain points, as at <i>a</i>, Fig. 70, +some of the bendings of the flinty chert are so sharp that +specimens might be broken off well fitted to serve as ridge-tiles +on the roof of a house. Although this chert could not have been +brittle as now, when first folded into this shape, it presents, +nevertheless, here and there, at the points of greatest flexure, +small cracks, which show that it was solid, and not wholly +incapable of breaking at the period of its displacement. The +numerous rents alluded to are not empty, but filled with chalcedony +and quartz.</p> + +<img src="../images/fig71.jpg" width="169" height="142" alt= +"Fig. 71: Bent and undulating gypseous marl." align="left"> + +<p>Between San Caterina and Castrogiovanni, in Sicily, bent and +undulating gypseous marls occur, with here and there thin beds of +solid gypsum interstratified. Sometimes these solid layers have +been broken into detached fragments, still preserving their sharp +edges (<i>g, g,</i> Fig. 71), while the continuity of the more +pliable and ductile marls, <i>m, m,</i> has not been +interrupted.</p> + +<img src="../images/fig72.jpg" width="179" height="67" alt= +"Fig. 72: Folded strata." align="right"> + +<p>We have already explained, Fig. 69, that stratified rocks have +usually their strata bent into parallel folds forming anticlinal +and synclinal axes, a group of several of these folds having often +been subjected to a common movement, and having acquired a uniform +strike or direction. In some disturbed regions these folds have +been doubled back upon themselves in such a manner that it is often +difficult for an experienced geologist to determine correctly the +relative age of the beds by superposition. Thus, if we meet with +the strata seen in the section, Fig. 72, we should naturally +suppose that there were twelve distinct beds, or sets of beds, No. +1 being the newest, and No. 12 the oldest of the series. But this +section may perhaps exhibit merely six</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 87">[ 87 ]</a></p> + +<img src="../images/fig73.jpg" width="235" height="158" alt="Fig. 73" +align="right"> + +<p>beds, which have been folded in the manner seen in Fig. 73, so +that each of them is twice repeated, the position of one half being +reversed, and part of No. 1, originally the uppermost, having now +become the lowest of the series.</p> + +<p>These phenomena are observable on a magnificent scale in certain +regions in Switzerland, in precipices often more than 2000 feet in +perpendicular height, and there are flexures not inferior in +dimensions in the Pyrenees. The upper part of the curves seen in +this diagram, Fig. 73, and expressed in fainter lines, has been +removed by what is called denudation, to be afterwards +explained.</p> + +<p><b>Fractures of the Strata and +Faults.</b>—Numerous rents may often be seen in rocks +which appear to have been simply broken, the fractured parts still +remaining in contact; but we often find a fissure, several inches +or yards wide, intervening between the disunited portions. These +fissures are usually filled with fine earth and sand, or with +angular fragments of stone, evidently derived from the fracture of +the contiguous rocks.</p> + +<p>The face of each wall of the fissure is often beautifully +polished, as if glazed, striated, or scored with parallel furrows +and ridges, such as would be produced by the continued rubbing +together of surfaces of unequal hardness. These polished surfaces +are called by miners “slickensides.” It is supposed that the lines +of the striæ indicate the direction in which the rocks were +moved. During one of the minor earthquakes in Chili, in 1840, the +brick walls of a building were rent vertically in several places, +and made to vibrate for several minutes during each shock, after +which they remained uninjured, and without any opening, although +the line of each crack was still visible. When all movement had +ceased, there were seen on the floor of the house, at the bottom of +each rent, small heaps of fine brick-dust, evidently produced by +trituration.</p> + +<p>It is not uncommon to find the mass of rock on one side of a +fissure thrown up above or down below the mass with which it was +once in contact on the other side. “This mode of displacement is +called a fault, shift, slip, or throw.” “The miner,” says Playfair, +describing a fault, “is often perplexed,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 88">[ 88 ]</a></p> + +<center><img src="../images/fig74.jpg" width="350" height="153" alt= +"Fig. 74: Faults."></center> + +<p>in his subterranean journey, by a derangement in the strata, +which changes at once all those lines and bearings which had +hitherto directed his course. When his mine reaches a certain +plane, which is sometimes perpendicular, as in A B, Fig. 74, +sometimes oblique to the horizon (as in C D, ibid.), he finds the +beds of rock broken asunder, those on the one side of the plane +having changed their place, by sliding in a particular direction +along the face of the others. In this motion they have sometimes +preserved their parallelism, as in Fig. 74, so that the strata on +each side of faults A B, C D, continue parallel to one another; in +other cases, the strata on each side are inclined, as in <i>a, b, +c, d</i> (Fig. 75), though their identity is still to be recognised +by their possessing the same thickness and the same internal +characters.”*</p> + +<center><img src="../images/fig75.jpg" width="350" height="141" alt= +"Fig. 75: E F, fault or fissure filled with rubbish, on each side of which the shifted strata are not parallel."> +</center> + +<p>In Coalbrook Dale, says Mr. Prestwich†, deposits of +sandstone, shale, and coal, several thousand feet thick, and +occupying an area of many miles, have been shivered into fragments, +and the broken remnants have been placed in very discordant +positions, often at levels differing several hundred feet from each +other. The sides of the faults, when perpendicular, are commonly +several yards apart, and are sometimes as much as 50 yards asunder, +the interval being filled with broken <i>débris</i> of the +strata. In following the</p> + +<p class="fnote">* Playfair, Illust. of Hutt. Theory, § +42.<br> +† Geol. Trans., second series. vol. v, p. 452.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 89">[ 89 ]</a></p> + +<p>course of the same fault it is sometimes found to produce in +different places very unequal changes of level, the amount of shift +being in one place 300, and in another 700 feet, which arises from +the union of two or more faults. In other words, the disjointed +strata have in certain districts been subjected to renewed +movements, which they have not suffered elsewhere.</p> + +<p>We may occasionally see exact counterparts of these slips, on a +small scale, in pits of loose sand and gravel, many of which have +doubtless been caused by the drying and shrinking of argillaceous +and other beds, slight subsidences having taken place from failure +of support. Sometimes, however, even these small slips may have +been produced during earthquakes; for land has been moved, and its +level, relatively to the sea, considerably altered, within the +period when much of the alluvial sand and gravel now covering the +surface of continents was deposited.</p> + +<p>I have already stated that a geologist must be on his guard, in +a region of disturbed strata, against inferring repeated +alternations of rocks, when, in fact, the same strata, once +continuous, have been bent round so as to recur in the same +section, and with the same dip. A similar mistake has often been +occasioned by a series of faults.</p> + +<center><img src="../images/fig76.jpg" width="329" height="200" alt= +"Fig. 76: Apparent alternations of strata caused by vertical faults."> +</center> + +<p>If, for example, the dark line A H (Fig. 76) represent the +surface of a country on which the strata <i>a, b, c</i> frequently +crop out, an observer who is proceeding from H to A might at first +imagine that at every step he was approaching new strata, whereas +the repetition of the same beds has been caused by vertical faults, +or downthrows. Thus, suppose the original mass, A, B, C, D, to have +been a set of uniformly inclined strata, and that the different +masses under E F, F G, and G D sank down successively, so as to +leave vacant</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 90">[ 90 ]</a></p> + +<p>the spaces marked in the diagram by dotted lines, and to occupy +those marked by the continuous lines, then let denudation take +place along the line A H, so that the protruding masses indicated +by the fainter lines are swept away--a miner, who has not +discovered the faults, finding the mass <i>a</i>, which we will suppose to +be a bed of coal four times repeated, might hope to find four beds, +workable to an indefinite depth, but first, on arriving at the +fault G, he is stopped suddenly in his workings, for he comes +partly upon the shale <i>b</i>, and partly on the sandstone <i> +c</i>; the same result awaits him at the fault F, and on reaching E +he is again stopped by a wall composed of the rock <i>d.</i></p> + +<p>The very different levels at which the separated parts of the +same strata are found on the different sides of the fissure, in +some faults, is truly astonishing. One of the most celebrated in +England is that called the “ninety-fathom dike,” in the coal-field +of Newcastle. This name has been given to it, because the same beds +are ninety fathoms (540 feet) lower on the northern than they are +on the southern side. The fissure has been filled by a body of +sand, which is now in the state of sandstone, and is called the +dike, which is sometimes very narrow, but in other places more than +twenty yards wide.* The walls of the fissure are scored by grooves, +such as would have been produced if the broken ends of the rock had +been rubbed along the plane of the fault.† In the Tynedale +and Craven faults, in the north of England, the vertical +displacement is still greater, and the fracture has extended in a +horizontal direction for a distance of thirty miles or more.</p> + +<p><b>Great Faults the Result of Repeated +Movements.</b>—It must not, however, be supposed that +faults generally consist of single linear rents; there are usually +a number of faults springing off from the main one, and sometimes a +long strip of country seems broken up into fragments by sets of +parallel and connecting transverse faults. Oftentimes a great line +of fault has been repeated, or the movements have been continued +through successive periods, so that, newer deposits having covered +the old line of displacement, the strata both newer and older have +given way along the old line of fracture. Some geologists have +considered it necessary to imagine that the upward or downward +movement in these cases was accomplished at a single stroke, and +not by a series of sudden but interrupted movements. They appear to +have derived this idea from a notion that the grooved walls</p> + +<p class="fnote">* Conybeare and Phillips Outlines, etc., p. +376.<br> +† Phillips, Geology, Lardner’s Cyclop., p. 41.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 91">[ 91 ]</a></p> + +<p>have merely been rubbed in one direction, which is far from +being a constant phenomenon. Not only are some sets of striæ +not parallel to others, but the clay and rubbish between the walls, +when squeezed or rubbed, have been streaked in different +directions, the grooves which the harder minerals have impressed on +the softer being frequently curved and irregular.</p> + +<center><img src="../images/fig77.jpg" width="323" height="161" alt= +"Fig. 77: Faults and denuded coal-strata, Ashby de la Zouch."> +</center> + +<p>The usual absence of protruding masses of rock forming +precipices or ridges along the lines of great faults has already +been alluded to in explaining Fig. 76, p. 89, and the same +remarkable fact is well exemplified in every coal-field which has +been extensively worked. It is in such districts that the former +relation of the beds which have been shifted is determinable with +great accuracy. Thus in the coal-field of Ashby de la Zouch, in +Leicestershire (see Fig. 77), a fault occurs, on one side of which +the coal-beds <i>a, b, c, d</i> must once have risen to the height +of 500 feet above the corresponding beds on the other side. But the +uplifted strata do not stand up 500 feet above the general surface; +on the contrary, the outline of the country, as expressed by the +line <i>z z</i>, is uniformly undulating, without any break, and +the mass indicated by the dotted outline must have been washed +away.*</p> + +<p>The student may refer to Mr. Hull’s measurement of faults, +observed in the Lancashire coal-field, where the vertical +displacement has amounted to thousands of feet, and yet where all +the superficial inequalities which must have resulted from such +movements have been obliterated by subsequent denudation. In the +same memoir proofs are afforded of there having been two periods of +vertical movement in the same fault--one, for example, before, and +another after, the Triassic epoch.†</p> + +<p>The shifting of the beds by faults is often intimately connected +with those same foldings which constitute the anti-</p> + +<p class="fnote">* See Mammatt’s Geological Facts, etc., p. 90 and +plate.<br> +† Hull, Quart. Geol. Journ., vol. xxiv, p. 318, 1868.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 92">[ 92 ]</a></p> + +<p>clinal and synclinal axes before alluded to, and there is no +doubt that the subterranean causes of both forms of disturbance are +to a great extent the same. A fault in Virginia, believed to imply +a displacement of several thousand feet, has been traced for more +than eighty miles in the same direction as the foldings of the +Appalachian chain.* An hypothesis which attributes such a change of +position to a succession of movements, is far preferable to any +theory which assumes each fault to have been accomplished by a +single upcast or downthrow of several thousand feet. For we know +that there are operations now in progress, at great depths in the +interior of the earth, by which both large and small tracts of +ground are made to rise above and sink below their former level, +some slowly and insensibly, others suddenly and by starts, a few +feet or yards at a time; whereas there are no grounds for believing +that, during the last 3000 years at least, any regions have been +either upheaved or depressed, at a single stroke, to the amount of +several hundred, much less several thousand feet.</p> + +<p>It is certainly not easy to understand how in the subterranean +regions one mass of solid rock should have been folded up by a +continued series of movements, while another mass in contact, or +only separated by a line of fissure, has remained stationary or has +perhaps subsided. But every volcano, by the intermittent action of +the steam, gases, and lava evolved during an eruption, helps us to +form some idea of the manner in which such operations take place. +For eruptions are repeated at uncertain intervals throughout the +whole or a large part of a geological period, some of the +surrounding and contiguous districts remaining quite undisturbed. +And in most of the instances with which we are best acquainted the +emission of lava, scoria, and steam is accompanied by the uplifting +of the solid crust. Thus in Vesuvius, Etna, the Madeiras, the +Canary Islands, and the Azores there is evidence of marine deposits +of recent and tertiary date having been elevated to the height of a +thousand feet, and sometimes more, since the commencement of the +volcanic explosions. There is, moreover, a general tendency in +contemporaneous volcanic vents to affect a linear arrangement, +extending in some instances, as in the Andes or the Indian +Archipelago, to distances equalling half the circumference of the +globe. Where volcanic heat, therefore, operates at such a depth as +not to obtain vent at the surface, in the form of an eruption, it +may nevertheless be conceived to give rise to upheavals, foldings, +and faults in</p> + +<p class="fnote">* H. D. Rogers, Geol. of Pennsylvania, p. 897.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 93">[ 93 ]</a></p> + +<p>certain linear tracts. And marine denudation, to be treated of +in the next chapter, will help us to understand why that which +should be the protruding portion of the faulted rocks is missing at +the surface.</p> + +<p><b>Arrangement and Direction of Parallel +Folds of Strata.</b>—The possible causes of the +folding of strata by lateral movements have been considered in a +former part of this chapter. No European chain of mountains affords +so remarkable an illustration of the persistency of such flexures +for a great distance as the Appalachians before alluded to, and +none has been studied and described by many good observers with +more accuracy. The chain extends from north to south, or rather +N.N.E. to S.S.W., for nearly 1500 miles, with a breadth of 50 +miles, throughout which the Palæozoic strata have been so +bent as to form a series of parallel anticlinal and synclinal +ridges and troughs, comprising usually three or four principal and +many smaller plications, some of them forming broad and gentle +arches, others narrower and steeper ones, while some, where the +bending has been greatest, have the position of their beds +inverted, as before shown in Fig. 73, p. 87.</p> + +<p>The strike of the parallel ridges, after continuing in a +straight line for many hundred miles, is then found to vary for a +more limited distance as much as 30°, the folds wheeling round +together in the new direction and continuing to be parallel, as if +they had all obeyed the same movement. The date of the movements by +which the great flexures were brought about must, of course, be +subsequent to the formation of the uppermost part of the coal or +the newest of the bent rocks, but the disturbance must have ceased +before the Triassic strata were deposited on the denuded edges of +the folded beds.</p> + +<p>The manner in which the numerous parallel folds, all +simultaneously formed, assume a new direction common to the whole +of them, and sometimes varying at an angle of 30° from the +normal strike of the chain, shows what deviation from an otherwise +uniform strike of the beds may be experienced when the geographical +area through which they are traced is on so vast a scale.</p> + +<p>The disturbances in the case here adverted to occurred between +the Carboniferous period and that of the Trias, and this interval +is so vast that they may have occupied a great lapse of time, +during which their parallelism was always preserved. But, as a +rule, wherever after a long geological interval the recurrence of +lateral movements gives rise to a new set of folds, the strike of +these last is different. Thus,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 94">[ 94 ]</a></p> + +<p>for example, Mr. Hull has pointed out that three principal lines +of disturbance, all later than the Carboniferous period, have +affected the stratified rocks of Lancashire. The first of these, +having an E.N.E. direction, took place at the close of the +Carboniferous period. The next, running north and south, at the +close of the Permian, and the third, having a N.N.W. direction, at +the close of the Jurassic period.*</p> + +<center><img src="../images/fig78.jpg" width="327" height="149" alt= +"Fig. 78: Unconformable junction of old red sandstone and Silurian schist at the Siccar Point, near St. Abb's Head, Berwickshire."> +</center> + +<p><b>Unconformability of +Strata.</b>— Strata are said to be unconformable when +one series is so placed over another that the planes of the +superior repose on the edges of the inferior (see Fig. 78). In this +case it is evident that a period had elapsed between the production +of the two sets of strata, and that, during this interval, the +older series had been tilted and disturbed. Afterwards the upper +series was thrown down in horizontal strata upon it. If these +superior beds, <i>d, d,</i> Fig. 78, are also inclined, it is plain +that the lower strata <i>a, a,</i> have been twice displaced; +first, before the deposition of the newer beds, <i>d, d,</i> and a +second time when these same strata were upraised out of the sea, +and thrown slightly out of the horizontal position.</p> + +<center><img src="../images/fig79.jpg" width="346" height="122" alt= +"Fig. 79: Junction of unconformable strata near Mons, in Belgium."> +</center> + +<p>It often happens that in the interval between the deposition of +two sets of unconformable strata, the inferior rock has not only +been denuded, but drilled by perforating shells. Thus, for example, +at Autreppe and Gusigny, near Mons, beds of an ancient (primary or +palæozoic) limestone, highly inclined, and often bent, are +covered with horizontal strata</p> + +<p class="fnote">* Edward Hull, Quart. Geol. Journ., vol. xxiv, p. +323.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 95">[ 95 ]</a></p> + +<p>of greenish and whitish marls of the Cretaceous formation. The +lowest, and therefore the oldest, bed of the horizontal series is +usually the sand and conglomerate, <i>a</i>, in which are rounded +fragments of stone, from an inch to two feet in diameter. These +fragments have often adhering shells attached to them, and have +been bored by perforating mollusca. The solid surface of the +inferior limestone has also been bored, so as to exhibit +cylindrical and pear-shaped cavities, as at <i>c</i>, the work of +saxicavous mollusca; and many rents, as at <i>b</i>, which descend +several feet or yards into the limestone, have been filled with +sand and shells, similar to those in the stratum <i>a.</i></p> + +<p><b>Overlapping +Strata.</b>—Strata are said to overlap when an upper +bed extends beyond the limits of a lower one. This may be produced +in various ways; as, for example, when alterations of physical +geography cause the arms of a river or channels of discharge to +vary, so that sediment brought down is deposited over a wider area +than before, or when the sea-bottom has been raised up and again +depressed without disturbing the horizontal position of the strata. +In this case the newer strata may rest for the most part +conformably on the older, but, extending farther, pass over their +edges. Every intermediate state between unconformable and +over-lapping beds may occur, because there may be every gradation +between a slight derangement of position, and a considerable +disturbance and denudation of the older formation before the newer +beds come on.</p> + +<br> + + +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch4.html"> +Chapter IV</a> / <a href="ch6.html">Chapter VI</a></small> +</body> +</html> + |