284 IMPERFECTION OF THE Chap. IX. 



are the result and measure of the degradation which 

 the earth's crust has elsewhere suffered. And what an 

 amount of degradation is implied by the sedimentary- 

 deposits of many countries! Professor Eamsay has 

 given me the maximum thickness, in most cases from 

 actual measurement, in a few cases from estimate, of 

 each formation in different parts of Great Britain ; and 

 this is the result : — 



Feet. 

 Palaeozoic strata (not including igneous beds) .. 57,154 



Secondary strata 13,190 



Tertiary strata 2,240 



— making altogether 72,584 feet ; that is, very nearly 

 thirteen and three-quarters British miles. Some of 

 these formations, which are represented in England 

 by thin beds, are thousands of feet in thickness on the 

 Continent. Moreover, between each successive formation, 

 we have, in the opinion of most geologists, enormously 

 long blank periods. So that the lofty pile of sedimen- 

 tary rocks in Britain, gives but an inadequate idea of 

 the time which has elapsed during their accumulation ; 

 yet what time this must have consumed! Good ob- 

 servers have estimated that sediment is deposited by the 

 great Mississippi river at the rate of only 600 feet in a 

 hundred thousand years. This estimate may be quite 

 erroneous ; yet, considering over what wide spaces very 

 fine sediment is transported by the currents of the 

 sea, the process of accumulation in any one area must 

 be extremely slow. 



But the amount of denudation which the strata have 

 in many places suffered, independently of the rate of 

 accumulation of the degraded matter, probably offers 

 the best evidence of the lapse of time. I remember 

 having been much struck with the evidence of denuda- 

 tion, when viewing volcanic islands, which have been 



