80 LYELL'S ELEMENTS OF GEOLOGY. 



Denudation of Stratified Rocks. 



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, neces- 

 sarily 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, per- 

 haps, 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 quarrjj.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 super- 

 ficial heaps of mud, sand, and gravel usually called alluvium, 

 may be likened to the rubbish of a quarry which has been re- 

 jected as useless by the workmen, or has fallen upon the road 

 between the quarry and the building, so as to lie scattered at ran- 

 dom 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 val- 

 leys on both sides of which the same strata are seen following 

 each other in the same order, and having the same mineral com- 

 position and fossil contents. We may observe for example, 

 several formations, as No. 1, 2, 3, 4, in the accompanying dia- 



gram (Fig 80.); No. 1. conglome- 

 rate, No. 2. clay, No. 3. grit, and 

 No. 4, limestone, each repeated in a 

 series of hills separated by valleys 

 varying in depth. When we exa- 

 mine the subordinate parts of these 

 four formations, we find, in like 



Valleys of denudation. j. ,. , , , 



a. alluvium. manner, distinct beds in each, cor- 



