18 F. Ameghino — Geology of Argentina. 



Glacial epocli ; for although it is certain that there are yet some who 

 believe not only in Miocene and Eocene, but also in Secondary Glacial 

 periods, it is likewise certain that Bobody pays attention to them. 



The Transverse Valleys of Patagonia. 



Having dispelled the ignorance as to the origin of the boulder 

 formation, this leads us naturally to determine the age of the for- 

 mation of the transverse valleys of Patagonia. It is evident that at 

 the bottom of the ancient sea in which the boulders were deposited, 

 these were scattered by the waters in a uniform manner over all the 

 submerged territory. The same may be said of the sheets of basalt ; 

 those also must have extended in a comparatively uniform manner, 

 without forming the steep cliffs which they exhibit to-day in the 

 river valleys. Darwin, speaking of the scai'ps of the valley of the 

 River Santa Cruz, said that the cliffs of basalt of the two opposite 

 sides were recognizable immediately as at one time forming a con- 

 tinuous bed. The same may be said of the beds of boulders which 

 in many parts form the opposite cliffs of the Patagonian valleys ; 

 those beds were continuous across the valleys, but there are now 

 no traces of them. 



It is evident that if the valleys had existed before the great marine 

 submergence referred to, they would have been completely filled 

 with marine deposits, which, even supposing they had been swept 

 away afterwards by the waters, would always have left numerous 

 traces buried in the innumerable angles of the slopes ; but as such 

 deposits do not exist, the inevitable conclusion is, that the formation 

 of the great transverse valleys of Patagonia was brought about by 

 great dislocations and gigantic faults at a comparatively recent 

 geological period, posterior to the boulder formation, and at the last 

 emergence of the land. 



The Pampean Formation. 



As deduced from these data, the Pampean Formation must be of 

 more recent date than the boulders of the Tehuelche Formation, 

 and thus it is in fact, though in Patagonia it exhibits small develop- 

 ment. We must, however, except the interior of the southern 

 extremity, in the region of the River Gallegos, where the deposits 

 of loess, superposed on the boulders, attain in some places a thickness 

 of 15 to 20 metres. With respect to the whole, since no fossils 

 have hitherto been found here, it cannot be affirmed categorically 

 that these deposits correspond with the Pampean Formation of the 

 province of Buenos Aires, and they may be of a much more recent 

 period. 



This doubt does not exist with respect to the isolated deposits met 

 with from the Eiver Coyle to the north, even at the bottom of the 

 valleys as happens at the mouth of the Santa Cruz, or in depressions 

 of the Tertiary formation of the coast like the deposit at San Julian 

 which yielded to Darwin the first remains of Macrauclienia. Others 

 occupy ancient bays of the ocean, and then we meet with bones of 

 land- mammals mixed with marine shells ; on his recent journey 



