TRANSACTIONS OF SECTION E. 933 



between the Ventana and Tandil Ranges. The tosca rock is almost everywhere 

 €11 evidence in the sloping plain which surrounds the Ventana and Curamalal 

 Ranges, and it is extensively exposed in the banks, and at the i-apids and falls in 

 all the little streams. I have noticed that, where these run over tosca, it has a 

 hardened surface for a depth of perhaps half an inch. This is due to the 

 hydraulic properties of the rock, which have in places been found so marked, 

 notably at Rosario, in Santa Fe, that a few years ago, Carrasco states, the beds 

 were worked for the manufacture of hydraulic cement. 



U'Orbigny, Darwin, Bravard, Sir Woodbine Parish, AVeddell, Heusser and 

 Claraz, and others, disagree as to the origin of the ' Pampean mud.' Darwin 

 wisely said ' it poured down from the north;' but the then paucity of geographical 

 knowledge regarding the interior of South America did not enable him to fix its 

 exact source and method of conveyance. Imbedded in the Pampean formation, 

 over widely extended areas, have been found the fossil remains of the mastodon, 

 megatherium, mylodon, and other gigantic animals, those from Rosario to the 

 south being mixed with shells of species still living in the neighbouring seas. 

 After the Pampean beds were formed, and their southern and eastern margin 

 began to emerge from the waters, the ocean along the shallow coast rolled up on 

 the gently -inclined plain quantities of shells, banks of which, miles in length, may 

 be seen to-day far inland, giving evidences by their curvature and general appear- 

 ance of having been piled up along an ancient coast line. 



Not far from Bahia Blanca I have ridden for miles along the top of one of 

 these embankments, about 20 feet high and 100 feet wide at the base. Most of 

 the shells had been broken by wave action, and were mixed with abundance of 

 rounded pumice, which probably floated down the Colorado and Negro from some 

 volcanic centre of the distant Andes. 



But a portion of the Pampean formation is still submerged, for toxcn rock may 

 be found throughout the length of the Plata estiiary forming the bottom of its 

 southern half. Thence it extends its eastern and southern margin, under water, 

 along the coast of Buenos Ayres, at least as far as Bahia Blanca. 



The savants whom I have named have presented us with abundant evidences 

 that the whole Pampean formation was once submerged. What appears to have 

 confused them is the finding of similar beds in widely-separated locahties, and at 

 elevations varying by thousands of feet. One may believe that, wherever in the 

 immense drainage area of the ancient Plata basin the conditions of rivers, lakes, 

 and inland seas were favourable to the distribution of silt from the mud-producing 

 rocks which margin the entire basin in such prodigious quantities, there the 'mud' 

 should be found ; and it is conceivable that even to-day, if Nature were to form a 

 lake by throwing a permanent dam across the entrance to an extensive valley 

 leading into that basin, the streams entering it would there deposit Pampean 

 mild. If this be true, there is no reason why such mud should not be found 

 resting immediately on crystalline rocks in places. The extent of any continuous 

 bed would alone be limited by the drainage area receiving the silt. The origin 

 and age of a deposit wherever found, be it at sea-level or 12,000 feet above, or be 

 it a square mile in area or 100,000 square miles, should be treated upon its indi- 

 vidual merits. Darwin alone appears to have entertained doubts as to tte con- 

 teniporaneous origin of all materials similar to the Pampean beds, attributing 

 their uniformity more to ' the similarity of the rocky framework of the continent.' 

 The Pampean Sea connected with the Atlantic Ocean between Uruguay and 

 the Tandil Sierra. It was probably about 1,400 miles in length, with an average 

 width of above 400 miles. Roughly estimated, its are a must have been about 600,000 

 square miles — say about two-thirds the size of the 3Iediterranean Sea. The area 

 of the ancient Mojos Lake was about 115,000 square miles, being seven-tenths that 

 of the Black Sea, and exceeding that of the five ' Great Lakes ' of North America, 

 which is 93,581 square miles. The relation of the Pampean Sea to the Mojos Lake 

 was similar to that of the Mediterranean to the Black Sea. Traces of it are still 

 observable, notably the great, low, flooded morass of Xarayas on the Upper Para- 

 guay River, and the ancient delta of the Parana, including the Ybara lagoon. The 

 Salina Grande was also an arm of it — a great inland fiord. The '^ 



