68 DENUDATION. [Ch. VI 



In the " Survey of Great Britain" (vol. i.), Professor Eamsay has shown 

 that the missing beds, removed from the summit of the Mendips, must have 

 been nearly a mile in thickness ; and he has pointed out considerable areas 

 in South "Wales and some of the adjacent counties of England, where 

 a series of primary (or palaeozoic) strata, not less than 11,000 feet in 

 thickness, have been stripped off. All these materials have of course 

 been transported to new regions, and have entered into the composition 

 of more modern formations. On the other hand, it is shown by obser- 

 vations in the same " Survey," that the palaeozoic strata are from 20,000 

 to 30,000 feet thick. It is clear that such rocks, formed of mud and 

 sand, now for the most part consolidated, are the monuments of denuding 

 operations, which took place on a grand scale at a very remote period in 

 the earth's history. For, whatever has been given to one area must al- 

 ways have been borrowed from another ; a truth which, obvious as it 

 may seem when thus stated, must be repeatedly impressed on the stu- 

 dent's mind, because in many geological speculations it is taken for 

 granted that the external crust of the earth has been always growing 

 thicker, in consequence of the accumulation, period after period, of sedi- 

 mentary matter, as if the new strata were not always produced at the 

 expense of pre-existing rocks, stratified or unstratified. By duly reflect- 

 ing on the fact, that all deposits of mechanical origin imply the trans- 

 portation from some other region, whether contiguous or remote, of an 

 equal amount of solid matter, we perceive that the stony exterior of the 

 planet must always have grown thinner in one place whenever, by acces- 

 sions of new strata, it was acquiring density in another. No doubt the 

 vacant space left by the missing rocks, after extensive denudation, is less 

 imposing to the imagination than a vast thickness of conglomerate or 

 sandstone, or the bodily presence as it were of a mountain-chain, with 

 all its inclined and curved strata. But the denuded tracts speak a clear 

 and emphatic language to our reason, and, like repeated layers of fossil 

 nummulites, corals or shells, or like numerous seams of coal, each based 

 on its under clay full of the roots of trees, still remaining in their natural 

 position, demand an indefinite lapse of time for their elaboration. 



No one will maintain that the fossils entombed in these rocks did not 

 belong to many successive generations of plants and animals. In like 

 manner, each sedimentary deposit attests a slow and gradual action, and 

 the strata not only serve as a measure of the amount of denudation 

 simultaneously effected elsewhere, but are also a correct indication of the 

 rate at which the denuding operation was carried on. 



Perhaps the most convincing evidence of denudation on a magnificent 

 scale is derived from the levelled surfaces of districts where large faults 

 occur. I have shown, in fig. 87, p. 63, and in fig. 91, how angular and 

 protruding masses of rock might naturally have been looked for on the 

 surface 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 determined with great ac- 



