IxX PROCEEDINGS OV TttE GEOLOGICAL SOCIETY. 



having collected round them, when they existed as islets or suhmarine 

 rocks, at a greater height than the hottom of the adjoining open sea ; 

 the cliifs having been subsequently worn during the elevation of the 

 whole country in mass*." 



The most remarkable feature to the geologist, of this great Pampean 

 formation, is the vast accumulation of the fossil remains of mammalia 

 which it contains, chiefly herbivorous, generally of great size, and be- 

 longing to extinct genera, some even to extinct families or orders, — 

 the Megatherium, Mylodon, Toxodon, Glyptodon, Scelidotherium, 

 Macrauchenia, Megalonyx and Mastodon. " The greater number of 

 them," Mr. Owen tells us, "are referable to the order which Cuvier has 

 called Edentata, and belong to that subdivision of the order which is 

 characterized by having perfect and sometimes complex molar teeth, 

 and an external osseous and tessellated coat of mail. The Megatherium 

 is the giant of this tribef ." 



Mr. Darwin has given many interesting descriptions of the localities 

 where these fossil bones have hitherto been found ; they are all be- 

 tween the 31st and 50th degrees of south latitude; and numerous 

 though the rem^ains already discovered have been, they can form only 

 a very small portion of what lie buried in the deposit ; for they have 

 as yet been almost exclusively found in the cliffs and steep banks of 

 rivers. *' I am firmly convinced," Mr. Darwin says, " that a deep 

 trench could not be cut in any line across the Pampas without inter- 

 secting the remains of some quadruped." The bones occur at all 

 depths, from the top to the bottom of the deposit ; he himself found 

 some close to the surface ; near Buenos Ayres a skeleton was disinterred 

 from a depth of 60 feet, and on the Parana two skeletons of the Mas- 

 todon were found only five or six feet above the base of the deposit. 



The theory of the formation of this vast extent of indurated mud 

 and calcareous concretions proposed by M. Alcide d'Orbigny in his 

 * Travels in South America,' viz. that it was produced by a vast and 

 sudden flood, — a debacle, is shown by Mr. Darwin to be inconsistent 

 with the various phsenomena which the deposit exhibits ; its structure, 

 its concretions, the horizontal layers of to sea rock, the absence of 

 granite and boulders, all indicating a slow and tranquil deposition, — to 

 say nothing of the improbability of the existence of a mass of fine 

 mud combined with carbonate of lime in a state fit for chemical segre- 

 gation, ready to be transported by the debacle, and suflicient in amount 

 to cover a space larger than the whole of France. The theory 

 which Mr. Darwin himself suggests appears a very intelligible and 

 probable explanation of the facts he describes. He supposes that the 

 materials of the Pampean formation were derived from the great area 

 of older rocks, igneous and sedimentary, in Brazil and the high country 

 to the north and west that surrounds the plains ; that they were trans- 

 ported by numerous streams and rivers and deposited in a vast bay, 

 the former estuary of the Plata, extending into the low country 

 of Banda Oriental and forming a part of the adjoining sea, in the same 

 manneraswe have seen that thedeltaof the Mississippi has beenformed. 

 This operation of transport, and deposit of similar materials, ap- 

 * Page 79. f Owen, Fossil Mammalia, Voyage of the Beagle, p. 15. 



