to the amount of several hundred, much less several thousand, feet.

Faulting on a very grand scale accompanied mountain formation, and appears to have occurred at the close of the action of the tangential thrust, or the from side to side force, which curved and upheaved the mass.

As a rule, mountains of different ages have the direction of their folds of strata along different lines of strike. The force producing the curves was directed at right angles to the line of strike.

After the cessation of the crushing action, dislocations occurred along the flanks of the mountain ranges, that is to say, along the lines of strike of the component strata, and strike faults were produced on a very grand scale.

Conformable and unconformable stratification.-When strata rest one upon the other horizontally or with the same dip they are conformable. But strata are said to be unconformable when one series is so placed over another that the planes of

Fig. 78.

Unconformable junction of old red sandstone and Silurian schist at the Sicarr Point, near St. Abb's Head, Berwickshire.

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 accumulated, in horizontal strata, upon it. If these superior beds, d d, fig. 78, are also inclined, it is plain that the lower strata, a a, have been twice displaced-first, before the deposition of the newer beds, d d, and a second time when the same strata were upraised out of the sea, and thrown slightly out of the horizontal position.

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 of greenish and whitish

marls of the Cretaceous formation (fig. 79). The lowest, and therefore the oldest, bed of the horizontal series is usually the

Fig. 79.

Junction of unconformable strata near Mons, in Belgium.

sand and conglomerate, a, 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 c, the work of saxicavous mollusca ; and many rents, as at b, which descend several feet or yards into the limestone, have been filled with sand and shells, simi. lar to those in the stratum a.

Overlap of strata.-Strata are said to overlap when an upper bed extends beyond the limits of a lower one.

Fig. 80.

Sediment

Overlap of strata.

e bede, Jurassic rocks. 1. Wealden. 2. Lower greensand. 3. Gault. 4. Upper greensand. 5. Chalk. (From Jukes-Brown, Phys. Geol. p. 388.)

spread over a region of subsidence has the area of deposit gradually increased, and the newest formed strata will overlap the next below them. Thus, as shore lines have subsided, shallowwater marine deposits have crept over the land, and as subsidence has progressed, deep-water deposits have come upon these last, and extended farther. Unconformity thus happens when one set of strata rest upon others with a different angle of dip, and lapse of time is indicated between the deposition of the last lamina of the lower and the first one of the upper stratum. When overlap is noticed, lapse of time and alterations in the physical geography of the area are inferred to have taken place between the deposition of successive strata; and this is more fully appreciated when erosion of a lower stratum is observed

beneath a super-imposed one. These lapses of time, both great and small, accompanied 'physical breaks.'

It is found that when two series of strata are unconformable or overlap, their fossils differ considerably. This is termed a palæontological break, and it may be slight or very nearly absolute. These breaks occur between all the great formations, and indicate the endings and beginnings of epochs, ages, and aspects of nature.

CHAPTER VI.

Denudation, disintegration of rocks and removal of products-Subaërial and marine denudation-Agents of denudation and their methods of action-Special action of wind and running water in the formation of Valleys, Cañons, Escarpments, Alluvium, and Loess-Littoral and submarine denudation-Great shoals-Needles, Inland sea cliffs-Results of denudation.

DENUDATION, which has been occasionally spoken of in the preceding chapters, is the disintegration of the earth's surface and the removal of the products by water in motion, whether of rivers or of the waves and currents of the sea, and by wind, and the consequent laying bare of some inferior rock. This operation has exerted an influence on the structure of the earth's crust as universal and important as sedimentary deposition itself; for denudation is the necessary antecedent of the production of all new strata of mechanical origin. The formation of every new deposit by the transport of sediment and pebbles, necessarily implies that there has been, somewhere else, a grinding down of rock into rounded fragments, sand, or mud, equal in quantity to the new strata. All deposition, therefore, except in the case of a shower of volcanic ashes, and the outflow of lava, and the growth of certain organic formations, is the sign of former superficial waste, or of that going on contemporaneously, and to an equal amount, elsewhere. The gain at cne point is no more than sufficient to balance the loss at some other. Here a hill has been lowered, there a ravine or valley has been deepened. Here the depth of the sea has been augmented by the removal of a sand-bank during a storm, there its bottom has been raised and shallowed by the accumulation on its bed of the same sand transported from the bank.

When we see a stone building, we know that somewhere, far or near, a quarry has been opened. The courses of stone in the building may be compared to successive strata, the quarry to a ravine or valley which has suffered denudation. As the strata,

like the courses of hewn stone, have been laid one upon another gradually, so the excavation both of the valley and quarry has been gradual. To pursue the comparison still farther, the superficial heaps of mud, sand, and gravel, usually called alluvium, may be likened to the rubbish of a quarry which has fallen upon the road between the quarry and the building, so as to lie scattered at random over the ground.

But we occasionally find in a conglomerate, large rounded pebbles of an older conglomerate, which had previously been derived from a variety of different rocks. In such instances we are reminded that strata have been formed by the deposition of denuded materials worn from older strata, and have been curved and elevated into hills and mountains. These in their turn have been worn down by the agents of denudation. In such cases it is evident that the same materials have been in very different conditions and positions, over and over again during the mutations which have affected the surface of the globe. Denudation and re-deposition have persisted ever since the earth's crust has been covered by an atmosphere, and has had its rivers and seas.

Denudation may be divided into subaërial and marine, and the agents which produce it are the sun's heat, cold, frost, the atmosphere, rain, rivers and the sea.

Subaërial denudation.-The sun acts on rocks by heating them, and when the component minerals expand differently with heat, and contract differently as they become cold, when the influence of the sun is at an end, disintegration proceeds. The sun dries clay, for instance, at the surface, and enables other agents to act. The alternations of heat and cold are attended by very remarkable results on rocks, brittleness being often produced. Prolonged cold, and especially with the aid of frost, moisture being present, is a great destroyer of the surface down to some depth, and the principal cause is the expansion of the water during the assumption of the crystalline state of ice. The atmosphere acts chemically and mechanically, and is assisted by the moisture it contains. Weathering of rocks by the carbonic acid of the air is assisted by the removal of the bicarbonates by rain and wind. The rapidity with which inscriptions on monuments in churchyards become effaced, when compared with similar records placed within the church, has often been pointed out as a striking illustration of the process of disintegration.

Professor Milne has shown how the sand-blast erodes the Arabian Wadys, scrubs the rocks, and removes the worn-off grains; and there are many examples of wind-borne and windwrecked rocks on every sea coast.

'Weathering' is often very conspicuous in crystalline rocks, such as granite and most volcanic rocks, which are composed of several mineral elements. Through the decomposition of the felspar and other minerals most liable to be chemically affected by air and rain, so hard rock as basalt sometimes crumbles to pieces, and it may be dug with a spade. Some of the most fertile districts in Italy and France owe their riches to the scoria and lava that once issued in a molten condition from the crater of Vesuvius and the volcanoes of Auvergne, destroying all the vegetation around, but which since then have cooled and crumbled into dust.

In desert regions, where no rain falls, or where, as in parts of the Sahara, the soil is so salt as to be without any covering of vegetation, clouds of dust and sand attest the power of the wind to cause the shifting of the unconsolidated or disintegrated rock.

In examining volcanic countries I have been much struck with the great superficial changes brought about by this power in the course of centuries. The higher peak of Madeira is about 6,050 feet above the sea, and consists of the skeleton of a volcanic cone now 250 feet high, the beds of which once dipped from a centre in all directions at an angle of more than 30°. The summit is formed of a dike of basalt with much olivine, fifteen feet wide, apparently the remains of a column of lava which once rose to the crater. Nearly all the scoriæ of the upper part of the cone have been swept away, those portions only remaining which were hardened by the contact or proximity of the dike. While I was myself on this peak on January 25, 1854, I saw the wind, though it was not stormy weather, removing sand and dust derived from the decomposing scoriæ. There had been frost in the night, and some ice was still seen in the crevices of the rock.

On the highest platform of the Grand Canary, at an elevation of 6,000 feet, there is a cylindrical column of hard lava, from which the softer matter has been carried away; and other similar remnants of the dikes of cones of eruption, attest the denuding power of the wind at points where running water could never have exerted any influence. The waste effected by wind, aided by frost and snow, may not be trifling, even in a single winter, and, when multiplied by centuries, may become indefinitely great.

Action of running water.-Rain carries off the products of denudation to a certain extent, and removes them to streams and rivers. Running water moving stones along erodes, and deposition occurs eventually. Perhaps the most striking illustrations of