298 THE LAPSE OF TIME. 



On the other hand, in all parts of the world the piles of 

 sedimentary strata are of wonderful thickness. In the Cor- 

 dillera, I estimated one mass of conglomerate at 10,000 feet; 

 and although conglomerates have probably been accumulated 

 at a quicker rate than finer sediments, yet from being 

 formed of worn and rounded pebbles ; each of which bears 

 the stamp of time, they are good to show how slowly the 

 mass must have been heaped together. Professor Ramsay 

 has given me the maximum thickness, from actual measure- 

 ment in most cases, of the successive formations 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 thir- 

 teen and three-quarters British miles. Some of the forma- 

 tions, 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, blank periods of enormous length. 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 eternity. 

 Nevertheless this impression is partly false. Mr. Croll, 

 in an interesting paper, remarks that we do not err "in 

 forming 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 representing several million years, the two produce a 

 totally different effect on the mind, and the figures are at 

 once pronounced too small. In regard to subaerial denuda- 

 tion, Mr. Croll shows, by calculating the known amount of 

 sediment annually brought down by certain rivers, relatively 

 to their areas of drainage, that 1,000 feet of solid rock, as 

 jt became gradually disintegrated, would thus be removed 

 from the mean level of the whole area in the course of six 

 million years. This seems an astonishing result, and some 

 considerations lead to the suspicion that it may be too large, 

 but if halved or quartered it is still very surprising. Few 

 of us, however, know what a million really means. Mr. 

 Croll gives the following illustration : Take a narrow strijp 



