Cleavage (geology)

“ If this happens chiefly or exclusively when the cleavage nearly coincides with the stratification, and happens rarely or not at all when the cleavage meets the strata at or about an angle of 45°, we may conclude that the cleavage plane is not perpendicular to any axis of antecedent pressure or tension, but is coincident with one of the two planes of maximum tangential movement.
Thus a delicate and critical inquiry is opened out for geologists who may have been trained to the accurate use of graduated instruments. ”

“ But both contortion and faulting may be due, on a small scale, to local causes, such as the intrusion of igneous rocks, the consolidation of strata, the falling in of old water-courses, &c. Cleavage is believed to have been caused by compression, such as the rocks might well be subjected to during great movements of the earth's crust. The particles of which the rock is composed are compressed in one direction, and of course are at the same time drawn out at right angles to the pressure. ”

“ In Tierra del Fuego, Darwin found striking evidence that the cleavage intersecting great masses of slate-rocks was quite independent of their original stratification, and could often, indeed, be seen cutting across it at right angles. He was also able to verify Sedgwick’s observation that, in some slates, glossy surfaces on the planes of cleavage arise from the development of new minerals, chlorite, epidote or mica, and that in this way a complete graduation from slates to true schists may be traced. ”

“ Thus, in a region like the Alps, where the strata often have been so completely folded as to be bent, so to say, back to back, the planes of cleavage, which are produced when the rocks can no longer yield to the pressure by bending, necessarily coincide with those of bedding. Still, even in these cases, if careful search be made in the vicinity, some minor flexure generally betrays the secret, and exhibits the cleavage structure cutting across that of bedding. ”

“ They urge that the platy minerals, mica and chlorite, are produced during the alteration of the rock, and can spread with ease in directions perpendicular to that of compression; they thus give rise to slaty cleavage at a late stage in the deformation of the rock. These authors, it will be seen, have developed one of Darwin's principal propositions, as to the close connexion between rock-cleavage and foliation, and, in opposition to Sorby, consider the platiness of the original constituents to be of less importance. ”

“ The clays, as above stated, are either laminated, or appear as vast uniform masses of sediment; bedded they can hardly ever be said to be, unless where interposed between beds of sandstone, or limestone. The iron stones, septaria, and "cone-in-cone" masses occur in the clays, in surfaces always parallel to the planes of stratification, and thus appear to mark periodical changes in the nature of the sediments; but this accumulation is generally the result of molecular attraction round organic bodies. ”

“ But if we investigate different mountain chains, we find gneiss, mica-schist, hornblende-schist, chlorite-schist, hypogene limestone, and other rocks, succeeding each other, and alternating with each other in every possible order. It is, indeed, more common to meet with some variety of clay-slate forming the uppermost member of a metamorphic series than any other rock; but this fact by no means implies, as some have imagined, that all clay-slates were formed at the close of an imaginary period when the deposition of the crystalline strata gave way to that of ordinary sedimentary deposits. ”

“ Over much of the northern states there is spread an unstratified stony clay called the drift. It often rests on sound rocks. It contains grains of sand, pebbles, and bowlders composed of many different minerals and rocks that the country rock cannot furnish. Hence the drift cannot have been formed by the decay of the rock of the region. A shale or limestone, for example, cannot waste to a clay containing granite pebbles. ”

“ We have already seen that the changes in plant and animal life point to a change from a hot climate, through a temperate, at last to arctic cold. Again, over the greater part of Northern England the rocks of the various geological periods are buried under sheets of tough clay, called boulder clay, for it is studded with boulders large and small, like raisins in a plum pudding. No flowing water forms such a deposit, but it is found to be just like the mass of clay with stones under the great glaciers and ice sheets of arctic regions ”

“ In such cases we sometimes discover that the more modern rock has been in part derived from the degradation of the older. Thus, for example, we may find chalk in one part of a country, and in another strata of clay, sand, and pebbles. If some of these pebbles consist of that peculiar flint, of which layers more or less continuous are characteristic of the chalk, and which include fossil shells, sponges, and foraminifera of cretaceous species, we may confidently infer that the chalk was the oldest of the two formations. ”

“ As a rule, fossils occur most abundantly and in the best state in clay-rocks, like shale; while in porous rocks, like sandstone, they are generally poorly preserved, and not of so frequent occurrence. One reason for this is, that clay-rocks are much less pervious than sandstone, and their imbedded fossils have consequently escaped in greater measure the solvent powers of percolating water. But there are other reasons for the comparative paucity of fossils in arenaceous strata, as we shall see presently. ”

“ If the Challenger hypothesis, that the red clay is the residue left by dissolved Foraminiferous skeletons, is correct, then all these deposits alike would be directly, or indirectly, the product of living organisms. But just as a silicious deposit may be metamorphosed into opal or quartzite, and chalk into marble, so known metamorphic agencies may metamorphose clay into schist, clay-slate, slate, gneiss, or even granite. ”

“ All these beds are traversed by numerous veins of quartz, and they alternate with each other without any regular order, the beds not forming a series passing from the coarse to the fine grained, but a fine clay slate often following close upon a coarse graywacke, and being succeeded by a similar rock. Here then we have an instance of a fact, of which the observations of every geologist will furnish many other examples, namely, the occurrence of clay slate among those rocks called rocks of transition, their alternation rendering this part of the fact indisputable. ”

“ In many districts, too, where the rock is richly fossiliferous, the broken joints of encrinites along with corals and shells may be seen crowded together by myriads, their hard skeletons protruding from the wasted rock in such a way as to show that the stone can contain very little else. By this means we often - 154 - learn that a limestone bed is nothing but an old sea-bottom, where the calcareous sediment was mainly derived from broken stone-lilies, corals, and shells, though if we break off a piece of the rock the internal fracture may show very little or no trace of any organic structure. ”

“ Those whose theoretical views lean to floating ice, believe that they can equally account for these phenomena, and can urge in support of their theory the occurrence of drift wood in the inland clay and boulder clay, and of sea-shells in the marginal clay and boulder clay, and the atmospheric decomposition of rock in the Pliocene period, as a source of the material of the clays, while to similar causes they can attribute the erosion of the deep valleys piled with the Post-pliocene deposits. ”

“ The unstratified portion of the drift consists chiefly of sheets of dense, stony clay called till, which clearly are the [419] ground moraines of ancient continental glaciers. Till is an unsorted mixture of materials of all sizes, from fine clay and sand, gravel, pebbles, and cobblestones, to large bowlders. The stones of the till are of many kinds, some having been plucked from the bed rock of the locality where they are found, and others having been brought from outside and often distant places. Land ice is the only agent known which can spread unstratified material in such extensive sheets. ”

“ The crystalline arrangement of the lime in the organic structures enables them to resist disintegration better than the general mechanically aggregated matrix of the rock. An experienced fossil collector will always search well such weathered surfaces, for he often finds there, delicately picked out by the weather, minute and frail fossils which are wholly invisible on a freshly broken surface of the stone. Many rocks weather with a thick crust, or even decay inwards for many feet or yards. ”

“ 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 DENUDATION, or that action of water which will be explained in the next chapter. ”

“ The outer limits of the deposits of clay and sand here referred to are known in some instances to be determined by ancient beaches and terraces. Such associations of deep and of shallow water deposits require special attention, as the study of one may assist in interpreting the significance of the other. The fine, evenly stratified clays frequently contain large angular bowlders, which appear to have been dropped from floating ice and to show an intimate connection between the401 ancient lakes and neighboring glaciers. ”

“ It is a pell-mell accumulation of silty clay, crammed with glaciated stones, amongst which are many fragments of red sandstone and some extra-Hebridean rocks, and interspersed through it occur also broken fragments of sea-shells. The marine deposits lying below are usually much confused and contorted, and here and there they are even violently commingled with the upper till. They show, generally, a most irregular surface under that accumulation, and are evidently only the wreck of what they must at one time have been. ”

“ Betwixt Sologne and the sea, patches are found to rest successively upon gneiss, clayslate, the coal-measures, Jura limestone, greenstone trap, chalk, and the upper beds of the eocene series. The pleiocene beds are not materially different in their lithological characters from those of the miocene group: blue clays, marls, and osseous breccias are among the prevailing strata; and siltings of sand and gravel, only distinguishable by their organic remains from the alluvia and superficial drifts of the current era. ”

“ Darwin has truly said that the fossiliferous rocks known to geologists consist, for the most part, of such as were formed when the bottom of the sea was subsiding. This downward movement protects the new deposits from denudation, and allows them to accumulate to a great thickness; whereas sedimentary matter, thrown down where the sea-bottom is rising, must almost invariably be swept away by the waves as fast as the land emerges. ”

“ If the transporting stream flows through valleys whose sides are formed of limestone, chalk, sandstone or other materials, these will become mixed with the clay, and to so great an extent has the mixing occurred that very few clays occur in a state even approximating to purity. The majority of clays are contaminated with iron oxide, lime compounds and free silica in such a fine state of division that it is impossible to purify them completely without [50] destroying the nature of the clay. ”

“ The material which passes through the finest sieve employed will contain all the true clay in the material; that is to say, the coarser portion will, as already mentioned, be devoid of the ordinary characteristics of clay. At the same time, this very fine material will seldom consist exclusively of clay, but will usually contain a considerable proportion of silt, extremely fine mineral particles and, in the case of calcareous clays, a notable proportion of calcium carbonate in the form of chalk or limestone particles. ”

“ For twenty years previously geologists were mostly content to accept on trust the suggestion of Professor Sedgwick, that crystalline forces had rearranged whole mountain masses so as to produce [Pg 37] a beautiful crystalline cleavage. In 1854 Professor Tyndall visited the quarries of Cumberland and North Wales, where the question of cleavage came prominently before him. When at Penrhyn Quarry he was told that the planes of cleavage were the planes of stratification lifted up by some convulsion into an almost vertical position. ”

“ Such disintegrations of rocks and separations of clay from sand have been gradually going on during the millions of years of the earth's existence, and are now proceeding, and have been the cause of the formation of the immense deposits of sandstone and clay now forming a part of the earth's strata. Such beds of clay may have been transferred by currents and streams from one locality to another, so that we must distinguish between primary and secondary deposits of clay. ”

“ Immediately below the clay the rock has disintegrated into meal where the cement between the limestone grains has been removed, while beneath this the laminae are split apart where the cement has been dissolved only along the planes of lamination where the stone is more porous. As these changes in the rock are greatest at the surface and diminish downward, we infer that they have been caused by agents working downward from the surface.
At certain points these agencies have been more effective than elsewhere. The upper rock surface is pitted. ”