The simplest hypothesis and the one which surmounts the greatest number of difficulties is that a mighty deluge overwhelmed man and beast in common ruin. A great geologist[6] says: "I argue that this destruction was caused by an invasion of the continent by water--a view which is completely _en rapport_ with the facts presented by the great Pampean deposit, which was clearly laid down by water. How otherwise can we account for this complete destruction and the homogeneousness of the Pampas deposits containing bones? I find an evident proof of this in the immense number of bones and of entire animals whose numbers are greatest at the outlets of the valleys, as Mr. Darwin shows. He found the greatest number of the remains at Bahia Blanca, at Bajada, also on the coast, and on the affluents of the Rio Negro, also at the outlet of the valley. This proves that the animals were floated, and hence were chiefly carried to the coast." But D'Orbigny seems to have erred in attempting to push his theory too far, for he insists that the great deluge not only destroyed the mammoths but at the same time created the Pampean plain. Nothing, however, can be more certain than that the lapse of countless ages was necessary to accumulate "the dust of continents to be." It is incredible that a great fragment of a continent was created _per saltum_. Darwin believes (and, it appears, rightly), "that the Pampean formation was slowly accumulated at the mouth of the former estuary of the Plata and in the sea adjoining it."[7] As we shall see, when we come to deal with Patagonia, the country was once a lake or sea, and the water system of South America was very different from what it now is, nor is there any difficulty in believing that the stupendous volume of the Parana waters (then even mightier than now) was able to wash down an accumulation of mud capable of making the sea into dry land. So much, then, for the Quaternary Pampean mud interlaced with the bones of giant animals. The Patagonian plain, however, is, in appearance at any rate, a different and much older formation, namely the Tertiary, an extensive gravel bed which possibly extends under the whole Quaternary deposit of the Pampa. But exposures occur of both varieties of this formation, _i.e._, the Patagonian and the Guaraman, in the banks of the Parana and elsewhere. It is supposed to have been contributed chiefly by Glacial action. [Illustration: THE RIVER URUGUAY.]