56 Geology of Houth America. 



the neighbouring coasts ; 2. None of the species have identical 

 analogues even in remote seas (M. d'Orbigny applies this con- 

 clusion to the basin of Paris itself, refusing to admit, along 

 vv^ith the greater number of paleontologists, that some of the 

 numerous fossils of that basin have living analogues) ; 3. The 

 genera, when they occur in the neighbouring seas, are now 

 in regions which are warmer and nearer the equator ; 4. A 

 great number of the genera met with in a fossil state are now 

 awanting in the neighbouring seas, and sometimes have even 

 ceased to exist. These different circumstances lead M. d'Or- 

 bigny to conclude, that the Patagonian and Chilian tertiary 

 formations both belong to the most ancient tertiary period, 

 whence it would result that they are contemporaneous, or 

 nearly so. This last distinction is important, for if it were 

 proved that the contemporaneity of the two formations must 

 have been absolute, it would likewise be necessary to conclude 

 with M. d'Orbigny, that during the period of the deposition 

 of these formations, the two seas where they were formed, 

 must have been separated in the same degree as they now 

 are, viz. those which wash the east and west coasts of Ame- 

 rica, and which, according to M. d'Orbigny, do not contain 

 analogous shells any more than the tertiary formations do. If, 

 on the contrary, as may be maintained, the facts observed in- 

 dicate only an approximative contemporaneity, the conclusion 

 relative to the existence of a continuous chain of mountains 

 between these two seas, still leaves room for desiring farther 

 information. 



The third of the great divisions which M. d'Orbigny dis- 

 tinguishes in the tertiary formations of South America, the 

 Painpean formation, differs essentially from the two series of 

 tertiary beds on which it reposes, in the simplicity of its com- 

 position, and, so to speak, in the unity of its mass. It is a 

 great bed of reddish argillaceous earth, generally containing 

 layers of pale brown calcareous concretions. These concre- 

 tions are hard where they are most compact, and are traversed, 

 as Mr Darwin has already observed, by small linear cavities, 

 which contribute to give them the characteristic aspect of 

 fresh water limestone.* They sometimes become so numerous 



♦ Darwin, Zoology of the Voyage of the Beagle ; Introduction, p. 4. 



