Gh. XIII. J IMPORTANCE OF FOSSIL SHELLS. 187 



temperature of certain zones cannot be materially raised or lowered 

 without others being immediately affected ; and the elevation or dimi- 

 nution in height of an important chain of mountains or the submergence 

 of a wide tract of land would modify the climate even of the antipodes. 



It will be observed that in the foregoing allusions to organic remains, 

 the testacea or the shell-bearing nioliusca are selected as the most useful 

 and convenient class for the purposes of general classification. In the 

 first place, they are more universally distributed through strata of every 

 age than any other organic bodies. Those families of fossils which are 

 of rare and casual occurrence are absolutely of no avail in establishing 

 a chronological arrangement. If we have plants alone in one group of 

 strata and the bones of mammalia in another, we can draw no conclusion 

 respecting the affinity or discordance of the organic beings of the two 

 epochs compared ; and the same may be said if we have plants and 

 vertebrated animals in one series and only shells in another. Although 

 corals are more abundant, in a fossil state, than plants, reptiles, or fish, 

 they are still rare when contrasted with shells, especially in the European 

 tertiary formations. The utility of the testacea is, moreover, enhanced 

 by the circumstance that some forms are proper to the sea, others to the 

 land, and others to freshwater. Rivers scarcely ever fail to carry down 

 into their deltas some land shells, together with species which are at 

 once fluviatile and lacustrine. By this means we learn what terrestrial, 

 freshwater, and marine species coexisted at particular eras of the past ; 

 and having thus identified strata formed in seas with others which origi- 

 nated contemporaneously in inland lakes, we are then enabled to advance 

 a step farther, and show that certain quadrupeds or aquatic plants, found 

 fossil in lacustrine formations, inhabited the globe at the same period 

 when certain fish, reptiles, and zoophytes lived in the ocean. 



Among other characters of the molluscous animals, which render 

 them extremely valuable in settling chronological questions in geology, 

 may be mentioned, first, the wide geographical range of many species ; 

 and, secondly, what is probably a consequence of the former, the great 

 duration of species in this class, for they appear to have surpassed iD 

 longevity the greater number of the mammalia and fish. Had each 

 species inhabited a very limited space, it could never, when imbedded in 

 strata, have enabled the geologist to identify deposits at distant points ; 

 or had they each lasted but for a brief period, they could have thrown 

 no light on the connection of rocks placed far from each other in the 

 chronological, or, as it is often termed, vertical series. 



Many authors have divided the European tertiary strata into three 

 groups — lower, middle, and upper ; the lower comprising the oldest 

 formations of Paris and London before-mentioned ; the middle those of 

 Bourdeaux and Touraine ; and the upper all those newer than the mid- 

 dle group. , 



When engaged in 1828 in preparing my work on the Principles of 

 Geology, I conceived the idea of classing the whole series of tertiary 

 strata in four groups, and endeavoring to find characters for each, >x- 



