112 PKINCIPLES OF CLASSIFICATION [Ch. X. 



ternative than to avail themselves of such breaks as still remain, or of 

 every hiatus in the chain of animated beings which is not yet filled up. 

 So in geology, we may be eventually compelled to resort to sections of 

 time as arbitrary, and as purely conventional, as those which divide the 

 history of human events into centuries. But in the present state of our 

 knowledge, it is more convenient to use the interruptions which still 

 occur in the regular sequence of geological monuments, as boundary 

 lines between our principal groups or periods, even though the groups 

 thus established are of very unequal value. 



The isolated position of distinct tertiary deposits in different parts of 

 Europe has been already alluded to. In addition to the difficulty pre- 

 sented by this want of continuity when we endeavor to settle the chrono- 

 logical relations of these deposits, another arises from the frequent 

 dissimilarity in mineral character of strata of contemporaneous date, 

 such, for example, as those of London and Paris before mentioned. The 

 identity or non-identity of species is also a criterion which often fails us. 

 For this we might have been prej)ared, for we have already seen, that 

 the Mediterranean and Red Sea, although within 10 miles of each other, 

 on each side of the Isthmus of Suez, have each their peculiar fauna ; 

 and a marked difference is found in the four groups of testacea now 

 living in the Baltic, English Channel, Black Sea, and Mediterranean, al- 

 though all these seas have many species in common. In like manner a 

 considerable diversity in the fossils of different tertiary formations, which 

 have been thrown down in distinct seas, estuaries, bays, and lakes, does 

 not always imply a distinctness in the times when they were pro- 

 duced, but may have arisen from climate and conditions of physical 

 geography wholly independent of time. On the other hand, it is now 

 abundantly clear, as the result of geological investigation, that difierent 

 sets of tertiary strata, immediately superimposed upon each other, con- 

 tain distinct imbedded species of fossils, in consequence of fluctuations 

 which have been going on in the animate creation, and by which in the 

 course of ages one state of things in the organic world has been substi- 

 tuted for another wholly dissimilar. It has also been shown that in 

 proportion as the age of a tertiary deposit is more modern, so is its 

 fauna more analogous to that now in being in the neighboring seas. It 

 is this law of a nearer agreement of the fossil testacea with the species 

 now living, which may often furnish us with a clue for the chronological 

 arrangement of scattered deposits, where we cannot avail ourselves of 

 any one of the three ordinary chronological tests ; namely, superposition, 

 mineral character, and the specific identity of the fossils. 



Thus, for example, on the African border of the Red Sea, at the 

 height of 40 feet, and sometimes more, above its level, a white calcare- 

 ous formation has been observed, containing several hundred species of 

 shells differing from those found in the clay and volcanic tuff of the 

 country round Naples, and of the contiguous island of Ischia. Another 

 deposit has been found at Uddevalla, in Sweden, in which the shells do 

 not agree with those found near Naples. But although in these three 



