foundations will again give way under it, so that it will fall again in the same direction. But, if the direction should change, the fact could not be discovered by observing the slickensides, because the last scoring would efface the lines of previous friction. In the present state of our ignorance of the causes of subsidence, an hypothesis which can explain the great amount of displacement in some faults, on sound mechanical principles, by 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.

124

CHAPTER VI.

DENUDATION, AND THE PRODUCTION OF ALLUVIUM.

Denudation defined Its amount equal to the entire mass of stratified deposits in the earth's crust - Horizontal sandstone denuded in Ross-shire Levelled surface of

countries in which great faults occur- Connexion of denudation and alluvial formations - Alluvium, how distinguished from rocks in situ Ancient alluviums called diluvium - Origin of these-Erratic blocks and accompanying gravel-Theory of their transportation by ice.

BEFORE We take leave of the aqueous or fossiliferous rocks, we have still to consider the alluvial formations. Denudation, which has been occasionally spoken of in the preceding chapters, is the removal of mineral matter by running water, whether by a river or marine current, and the consequent laying bare of some inferior rock. Geologists are, perhaps, seldom in the habit of reflecting that this operation is the inseparable accompaniment of the production of all new strata of mechanical origin. The transport of sediment and pebbles, to form a new deposit, 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. The gain at one point has merely been sufficient to balance the loss at some other. A ravine, perhaps, has been

excavated, or a valley deepened, or the bed of the sea has, by successive upheaval, been exposed to the power of the waves, so that part of the superior covering of the earth's crust has been stripped off, and thus rocks, previously hidden, have been denuded.

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 have 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 cf a quarry which has been rejected as useless by the workmen, or has fallen upon the road between the quarry and the building, so as to lie scattered at random over the ground.

If, then, the entire mass of stratified deposits in the earth's crust is at once the monument and measure of the denudation which has taken place, on how stupendous a scale ought we to find the signs of this removal of transported materials in past ages! Accordingly, there are different classes of phenomena, which attest in a most striking manner the vast spaces left vacant by the erosive power

of water. I may allude first, to those valleys on both sides of which the same strata are seen following each other in the same order, and having the same mineral composition and fossil contents. We may observe for example, several formations, as Nos. 1, 2, 3, 4, in the accompanying diagram (Fig. 80.); No. 1. conglo4 merate, No. 2. clay, No. 3.

Fig. 80.

Valleys of denudation.

a. alluvium.

2

grit, and No. 4. limestone, each repeated in a series of hills separated by valleys varying in depth.

When we examine the subordinate parts of these four formations, we find, in like manner, distinct beds in each, corresponding, on the opposite sides of the valleys, both in composition and order of position. No one can doubt that the strata were originally continuous, and that some cause has swept away the portions which once connected the whole series. A torrent on the side of a mountain produces similar interruptions, and when we make artificial cuts in lowering roads, we expose, in like manner, corresponding beds on either side. in nature, these appearances occur in mountains several thousand feet high, and separated by intervals of many miles or leagues in extent, of which a grand exemplification is described by Dr. MacCulloch, on the north-western coast of Rossshire, in Scotland.*

* Western Islands, vol. ii. p. 89. pl. 31. fig. 4.

But

Fig. 81.

Suil Veinn.

Coul beg.

Coul more.

Denudation of red sandstone on north-west coast of Ross-shire.

The

The fundamental rock of that country is gneiss, in disturbed strata, on which beds of nearly horizontal red sandstone rest unconformably. latter are often very thin, forming mere flags, with their surface distinctly ripple-marked. They end abruptly on the declivities of many insulated mountains, which rise up at once to the height of about 2000 feet above the gneiss of the surrounding plain or table-land, and to an average elevation of about 3000 feet above the sea, which all their

summits generally attain. The base of gneiss varies in height, so that the lower portions of the sandstone occupy different levels, and the thickness of the mass is various, sometimes exceeding 3000 feet. It is impossible to compare these scattered portions without imagining that the whole country has once been covered with a great body of sandstone, and that masses from 1000 to more than 3000 feet in thickness have been removed.

But perhaps the most convincing evidence of denudation on a magnificent scale is derived from the levelled surface of many districts in which large faults occur. I have already shown, in Fig. 78, p. 119, and in Fig. 82, how angular and pro