MAMMALIA OF THE MIOCENE PERIOD. 77 „ 



Mammalia of the Miocene Period. 



The second or Miocene System of Tertiary Deposites, 

 contains an admixture of the extinct genera of lacustrine 

 mammalia, of the first or Eocene series, with the earliest 



that flow from the Lagodi Tartaro, near Rome, and t!ie hot springs of San 

 Filippo, on the borders of Tuscany, are well-known examples of this pheno- 

 menon. These existing operations afford a nearly certain explanation of the 

 origin of extensive beds of limestone in fresh-water lakes of the tertiary pe- 

 riod, where we know them to have been formed during seasons of intense 

 volcanic activity. They seem also to indicate the probable agency of 

 thermal waters in the formation of still larger calcareous deposites at the 

 bottom of the sea, during preceding periods of the secondary and transition 

 series. 



It is a difficult problem to account for the source of the enormous masses 

 of carbonate of lime that compose nearly one-eighth part of the superficial 

 crust of the globe. Some have referred it entirely to the secretions of marine 

 animals ; an origin to which we must obviously assign those portions of 

 calcareous strata which are composed of comminuted shells and corallines : 

 but, until it can be shown that these animals have the power of forming lime 

 from other elements, vvc must suppose that they derived it from the sea, 

 cither directly, or through the medium of its plants. In cither case, it re- 

 mains to find the source whence the sea obtained, not only these supplies 

 of carbonate of lime for its animal iniiabilants, but also the still larger 

 quantities of the same substance, that have been precipitated in the form of 

 calcareous strata. 



We cannot suppose it to have resulted, like sands and clays, from tiic me- 

 chanical detritus of rocks of the granitic scries, because the quantity of lime 

 these rocks contain, bears no proportion to its large amount among the de- 

 rivative rocks. The only remaining hypothesis seems to be, that lime was 

 eontinually introduced to lakes and seas, by water that had percolated rocks 

 through which calcareous earth was disseminated. 



Although carbonate of lime occurs not in distinct masses among rocks of 

 igneous origin, it forms an ingredient of lava and basalt, and of various kinds 

 of trap rocks. The calcareous matter thus dispersed through the substance 

 of these volcanic rocks, seems to afford a magazine from which percolating 

 water, charged with carbonic acid gas, may, in the lapse of ages, have de- 

 rived sufficient carbonate of lime to form all the existing strata of limestone, 

 by successive precipitates at the bottom of ancient lakes and seas. Mr. De 

 la Bcche states the quantity of lime in granite composed of two-fifths quartz, 

 two-fifths felspar, and one-fifth mica, to be 0.37 ; and in greenstone, com- 

 posed of equal parts of felspar and hornblende, to be 7.29. (Geol. Researches, 



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