186 



THE GEOLOGIST. 



has to stand its chance of being obliterated by heat, or washed away 

 by water. 



" As all sedimentary strata are deposited from water, it follows that 

 for every cubic yard deposited a cubic yard must be denuded from 

 some other place ; and as the sedimentary rocks are much more com- 

 mon at the surface, and generally softer than the igneous ones, the 

 burden of supplying the sediment falls chiefly on them. We may 

 therefore feel sure that during any one period nearly as many fossili- 

 ferous strata are obliterated as are formed. In fact the power of 

 denudation is so great, that Mr. Darwin and many other geologists 

 think that only deposits formed during periods of subsidence are thick 

 enough to resist its foice, so that many species, and even genera, that 

 had but a limited range may have been swept away, and all record 

 of their existence destroyed. 



This denudation added to the periods of repose will make the 

 intervals between strata represent collectively far more time than the 

 strata themselves, and we have many proofs that this is true in the 

 numerous foreign strata that are intermediate in age to some of ours, 

 in unconform ability of stratification,* and in the abrupt change in 

 the organic remains of consecutive formations. 



Three-fourths of the globe are covered with water, therefore three- 

 fourths of the sti ata that remain are hidden from us ; and the other 

 fourth has to be divided among all the formations that have as yet- 

 been recognised, for we can but examine the surface. Of the fourth 

 that is accessible, not more than a fifth has been geologically explored ;t 

 and that only where sections happen to exist. We must also re- 

 member that large tracts of country, shown as Silurian, Devonian, &c, 

 on our maps, are covered so deeply with drift and alluvium that they 

 never have been, and perhaps never will be examined. 



For all these reasons the geological record must be very imperfect, 

 and when we examine it we find such to be the case ; for we have no 

 reason to suppose that the globe was less thickly inhabited in old 

 times than now : on the contrary, when we find fossils at all they 

 are generally in great abundance ; yet the number in any one forma- 

 tion is almost as nothing compared to the number of living animals 

 and plants. 



Mr. Darwin has justly observed " that in order to get a perfect 

 gradation between two forms in the upper and lower parts of the 

 same formation the deposit must have gone on accumulating for a 

 very long period, in order to have given time for the slow process of 

 variation, hence the deposit will generally have to be a very thick 

 one ; and the species undergoing modification will have had to 

 live on the same area throughout this time. But we have 



* The conformability of one stratum to another is no proof of its close se- 

 quence 5 for strata are sometimes conformable in one place, and unconformable 

 in another. 



^ t By explored I mean the age of its strata well made out, not simply guessed 



