82 



LYELL'S ELEMENTS OF GEOLOGY. 



Denudation. 



Fig. 82. 



Faults and denuded coal strata. 



de la Zouch. 



one side of which the coal beds abed rise to the height of 500 

 feet above the corresponding beds on the other side. But the 

 upHfted strata do not stand up 500 feet above the general surface; 

 on the contrary, the outline of the country, as expressed by the 

 line z z, is uniform and unbroken, and the mass indicated by the 

 dotted outline must have been washed away.* There are proofs 

 of this kind in some level countries, where dense masses of strata 

 have been cleared away from areas several hundred square miles 

 in extent. 



In the Newcastle coal district, it is ascertained that faults occur 

 in which the upward or downward movement could not have been 

 less than 140 fathoms, which, had they affected equally the con- 

 figuration of the surface to that amount, would produce mountains 

 with precipitous escarpments near 1000 feet high, or chasms of 

 the like depth ; yet is the actual level of the country absolutely uni- 

 form, affording no trace whatever of subterraneous disturbance. f 



The ground from which these materials have been removed, 

 is usually overspread with heaps of sand and gravel, formed out of 

 the ruins of the very "rocks which have disappeared. Thus, in the 

 districts above alluded to, rounded and angular fragments occur 

 of hard sandstone, limestone, and ironstone, with a small quan- 

 tity of the more destructible shale,' and even rounded pieces of 

 coal, the form of these relics pointing to water as the denuding 

 agent. 



In geological descriptions we often read of "alluvium" and 

 " diluvium," as opposed to " regular strata," or " fixed rocks," 

 or " rocks in situ." It will be useful, therefore, to explain these 

 terms. At the surface there is commonly a layer of vegetable 

 mould, derived partly from decayed plants, and partly caused by 

 the castings of earth-worms, which are continually sifting the 



*See Mammal's Geological Facts, (fee. p. 90. and plate, 

 t Cony beare's Report to Brit. Assoc. 1832. p. 381. 



