Chap. IX. THE LAPSE OF TIME. 271 



Oil the other Imnd, in all parts of the world the piles of 

 Kcdimentary strata arc of wonderful thickness. In the Cordil- 

 lera I estimated one mass of cono:lomcrate at ten thousand 

 feet ; and althou_Q;h conf2;lomerates have probably been accu- 

 nuilated at a quicker rate than liner sediments, yet from being 

 formed of worn and rounded pebbles, each of which bears the 

 stamp of time, they are f:^ood to show how slowly tlic mass 

 must have been heaped to<j;'ether. Prof. Ramsay has <;iven me 

 the niaxiir.um thickness, from actual measurement in most 

 cases, of the successive formations in dijf'ercnt parts of Great 

 Britain; and this is the result: 



Feet. 



Paleozoic BtratA (not including? igneous beds) 57,154 



Secondary strata 13.100 



Tertiary strata 2,240 



— makinfi;' altogether 72,584 feet ; that is, very nearly thirteen 

 and three-(iuarters British miles. Some of the formations, 

 Avhich 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 o])inion of most geolo- 

 gists, enormously long blank periods. So that the lofty pile 

 of sedimentary rocks in Britain gives but an inadequate idea 

 of the time which has elapsed during their accumulation. The 

 consideration of these various facts impresses the mind almost 

 in the same manner as does the vain endeavor to grapple with 

 the idea of eteniity. 



Nevertheless this impression is partly false. Mr. Croll, in 

 a most interesting paper, remarks that we do not err " in form- 

 ing too great a conception of the length of geological periods," 

 but in estimating them by years. When geologists look at 

 large and complicated phenomena, and then at the figures rep- 

 resenting several million years, the two produce a totally dif- 

 ferent effect on the mind, and the figures are at once pro- 

 nounced to be too small. But in regard to denudation, Mr. 

 Croll shows, by calculating the known amount of sediment an- 

 nually brought down by certain rivers, relatively to the areas 

 of drainage, that 1,000 feet of rock, disintegrated through sub- 

 ai'rial agenci(>s, would thus be removed from the mean level of 

 the whoh* area in the course of six million vears. This seems 

 an astonishing result, and some considerations lead to the sus- 

 picion that it may be much too large, but even if halved or 

 (juartered it is still very surprising. Few of us, however, know 

 what a million really means: Mr. Croll gives the following 



