Ch. VL] 



DENUDATION" OF STRATIFIED ROCKS. 



67 



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 re- 

 moval 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 

 Fig. 89. formations, as Nos. 1, 2, 3, 4, in the accom- 



panying diagram (fig. 89) ; No. 1 conglom- 

 erate, 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 examine the subordinate parts of 

 these four formations, we find, in like man- 

 ner, 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 man- 

 ner, corresponding beds on either side. But in nature, these appearances 

 occur in mountains several thousand feet high, and separated by inter- 

 vals of many miles or leagues in extent, of which a grand exemplifica- 

 tion is described by Dr. MacCulloch, on the northwestern coast of Ross- 

 shire, in Scotland.* 



Fig. 90. 



Valleys of denudation. 

 a. alluvium. 



Suil Yeinn. 



X\^./Y ITT^ 



Coul beg. Coul m 



5^^S?N 



^s \ / -\_ | 



=^777-^-r^l^^r^x : ^o 



Denudation of red sandstone on northwest coast of Eoss-shire. (MacCulloch.) 



The fundamental rock of that country is gneiss, in disturbed strata, on 

 which beds of nearly horizontal red sandstone rest unconformably. The 

 latter are often very thin, forming mere flags, with their surfaces dis- 

 tinctly 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 sum- 

 mits 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 and detached portions without imagining 

 that the whole country has once been covered with a great body of sand- 

 stone, and that masses from 1000 to more than 3000 feet in thickness have 

 been removed. 



* Western Islands, vol. ii. p. 93, pL 31, fig. 4. 



