Ch. Xin."l OF TERTIARY FORMATIONS. 1§5 



For this we might have been prepared, 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 different 

 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, e. g. in the Bay of Baias. 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 

 cases there may be scarcely a single shell common to the three different 

 deposits, we do not hesitate to refer them all to one period (the Post- 

 Pliocene), because of the very close agreement of the fossil species in 

 every instance with those now living in the contiguous seas. 



To take another example, where the fossil fauna recedes a few steps 

 farther back from our own times. We may compare, first, certain 

 beds at the eastern base of Etna near Trezza, hereafter to be men- 

 tioned ; secondly, others of fluvio-marine origin near Norwich ; and, 

 lastly, a third set often rising to considerable heights in Sicily, and we 

 discover that in every case more than three-fourths of the shells agree 

 with species still living, while the remainder are extinct. Hence we may 

 conclude that all these, greatly diversified as are their organic remains, 

 belong to one and the same era, or to a, period immediately antecedent 

 to the Post-Pliocene, because there has been time in each of the areas 

 alluded to for an equal or nearly equal amount of change in the marine 

 testaceous fauna. Contemporaneousness of origin is inferred in these 



