ORIGIN AND AGE OF THE SUNS HEAT. 273 



after all indications of dislocation had been effaced at 

 the surface. 



But it must be observed that the total quantity of 

 rock which has been removed from the 'present surface 

 of the land is evidently small in proportion to the total 

 quantity removed during the past history of our globe. 

 For those thousands and thousands of feet of rock 

 which have been denuded were formed out of the 

 waste of previously existing rocks, just as these had 

 been formed out of the waste of yet older rock-masses. 

 In short, as a general rule, the rocks of one epoch have 

 been formed out of those of preceding periods, and go 

 themselves to form those of subsequent epochs. 



In many of the cases of enormous denudation to 

 which we have referred, the erosion has been effected 

 during a limited geological epoch. We have, for 

 example, seen that upwards of a mile in thickness of 

 Carboniferous rock has been denuded in the area of 

 the Pentlands. But the Pentlands themselves, it can 

 be proved, existed as hills, in much their present form, 

 before the Carboniferous rocks were laid down over 

 them ; and as they are of Lower Old Red Sandstone 

 age, and have been formed by denudation, they must 

 consequently have been carved out of the solid rock 

 between the period of the Old Keel Sandstone and the 

 beginning of the Carboniferous age. This affords us 

 some conception of the immense lapse of time 

 represented by the Middle and Upper Old Red 

 Sandstone periods. 



Again, in the case of the great fault separating the 

 Silurians of the south of Scotland from the Old Red 

 Sandstone tracts lying to the north, a thickness of the 

 latter strata of probably more than a mile, as we have 

 seen, must have been removed from the ground to the 

 south of the fault before the commencement of the 



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