Chap. x. The Lapse of Time. 269 



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 the 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, 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 

 endeavour 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 

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

 sediment annually brought down by certain rivers, relatively to their 

 areas of drainage, that 1000 feet of solid rock, as it became gradu- 

 ally 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 even 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 

 strip of paper, 83 feet 4 inches in length, and stretch it along the 

 wall of a large hall ; then mark off at one end the tenth of an inch. 

 This tenth of an inch will represent one hundred years, and the 

 entire strip a million years. But let it be borne in mind, in relation 

 to the subject of this work, what a hundred years implies, repre- 

 sented as it is by a measure utterly insignificant in a hall of the 

 above dimensions. Several eminent breeders, during a single life- 

 time, have so largely modified some of the higher animals, which 

 propagate their kind much more slowly than most of the lower 

 animals, that they have formed what well deserves to be called a 

 new sub-breed. Few men have attended with due care to any one 

 strain for more than half a century, so that a hundred years repre- 

 sents the work of two breeders in succession. It is not to be sup- 



