PART I. CHAPTER VI. 81 



Denudation. 



responding, on the opposite sides of the valleys, both in compo- 

 sition 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. But 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. MacCul- 

 loch, on the north-western coast of Ross-shire, in Scotland.* 



Fig: 81. 



SuilVeinn. CoulUg, Coutmore. 



Denudation of red sandstone on north-west coast of JRoss-skire. 



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 surface distinctly ripple-marked. They end ab- 

 ruptly 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 gene- 

 rally 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 ima- 

 gining 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. 77, and in Fig. 82, how angular and protruding 

 masses of rock might naturally have been looked for on the sur- 

 face immediately above great faults, although in fact they rarely 

 exist. This phenomenon may be well studied in those districts 

 where coal has been extensively worked, for there the former 

 relation of the beds which have shifted their position may be de- 

 termined with great accuracy. Thus in the coal field of Ashby 

 de la Zouch, in Leicestershire (see Fig. 82.), a fault occurs, on 



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



