290 Zoology of the Voyage of the Beagle. 



The occurrence of certain species of marine shells, identi- 

 cal with species existing in the neighbouring sea, in the same 

 formation as that containing the fossil Mammalia, is particu- 

 larly noticed by Mr. Darwin, as a proof of the very recent e- 

 poch to which these fossiliferous beds should be referred. — 

 Upon this point, however, and upon the manner in which the 

 remains of land animals were introduced in such abundance 

 into these marine deposits, we will give the author's views in 

 his own words. 



*** " We may feel certain, that at a period not very remote, a great bay 

 occupied the area both of the Pampas, and of the lower parts of Banda 

 Oriental. Into this bay the rivers which are now united in the one great 

 stream of the Plata, must formerly have carried down, (as happens at the 

 present day), the carcasses of the animals inhabiting the surrounding 

 countries ; and their skeletons would thus become entombed in the estuary- 

 mud which was then tranquilly accumulating. Nothing less than a long 

 succession of such accidents can account for the vast number of remains 

 now found buried. As their exposure has invariably been due to the intersec- 

 tion of the plain by the banks of some stream, it is not making an extravagant 

 assertion, to say, that any line whatever drawn across the Pampas, would 

 probably cross the skeleton of some extinct animal. 



"At Bajada, a passage, as I have stated, may be traced upwards from the 

 beds containing marine shells, to the estuary-mud with the bones of land 

 animals. In another locality a bed of the same mineralogical nature with 

 the Pampas deposit, underlies clay containing large oysters and other shells, 

 apparently the same with those at Bajada. We may, therefore, conclude 

 that at the period when the Area, Venus, and Oyster were living, the phy- 

 sical condition of the surrounding country was nearly the same, as at the 

 time when the remains of the mammalia were embedded; and therefore 

 that these shells and the extinct quadrupeds probably either co-existed, or 

 that the interval between their respective existences was, in a geological 

 point of view, extremely short. In this part of South America there is rea- 

 son to believe that the movements of the land have been so regular, that 

 the period of its elevation may be taken as an element in considering the 

 age of any deposit. The circumstance, therefore, that the beds immediately 

 bordering the Plata, contain very nearly the same species of molluscs, with 

 those now existing in the neighbouring sea, harmonizes perfectly with the 

 more ancient (though really modern) tertiary character of the fossils un- 

 derlying the Pampas deposit at Bajada, situated at a greater height, and 

 at a considerable distance in the interior. I feel little doubt that the final 

 extinction of the several large quadrupeds of La Plata did not take place, 

 until the time when the sea was peopled with all, or nearly all, its present 

 inhabitants." Page 6. 



"In another part of the bay, called Punta Alta, about eighteen miles from 

 Monte Hermoso, a very small extent of cliff, about twenty feet high, is ex- 

 posed. The lower bed, seen at ebb tide, extends over a considerable area ; 

 it consists of a mass of quartz shingle, irregularly stratified, and divided by 

 curved layers of indurated clay. The pebbles are cemented together by 

 calcareous matter, which results, perhaps, from the partial decomposition of 

 numerous embedded shells. In this gravel the remains of several gigantic 

 animals were extraordinarily numerous. The cliff, in the part above high 

 water mark, is chiefly composed of a reddish indurated argillaceous earth ; 

 which either passes into, or is replaced by, the same kind of gravel as that 

 on which the whole rests. The earthy substance is coarser than that at 



