F. Ameghino — Geology of Argentina. 5 



Andes, it was constituted exclusively by the marine Patagonian 

 Formation, on the age of which the most varied opinions were 

 expressed, although all agreed in referring it to the Tertiary series. 

 This uniformity was interrupted only at intervals by great sheets 

 of basalt which appeared sometimes at the surface, sometimes below 

 the boulder-deposit. 



The results of the travels of my brother, Carlos Ameghino, who 

 since the year 1887 has dedicated himself exclusively to the geological 

 and paleeontological exploration of the regions of Patagonia, have 

 completely changed this view. These explorations, which have 

 revealed the former existence in the southern extremity of South 

 America of various truly remarkable extinct faunas (as the most 

 illustrious palaeontologists of the present day term them), have 

 also demonstrated to us that the Territory of Patagonia is not of 

 so uniform a geological constitution as was said, since it compi-ises 

 formations of different periods, marine and terrestrial or fluviatile, 

 the latter predominating over the former. 



Hitherto he has oxAy published a review of his two first voyages,^ 

 but he is preparing a detailed description of all his explorations, 

 and although on my part I liave given some short notices of his 

 discoveries on different occasions, it has appeared to me useful to 

 attempt a sketch here of the principal geological results obtained. 



Little can be said concerning the most ancient sedimentary 

 formations, since these are only visible at certain points where 

 they have been raised by the eruption of large masses of igneous 

 rocks, principally porphyries, which have placed them within range 

 of discovery ; this happens in the central part of the Territory of 

 Chubut, and also in the region of the Deseado and various other 

 places. 



Supposed Jurassic Strata. 



The most ancient sedimentary deposits, which rest immediately 

 upon these eruptive rocks, appear in the upper course of the River 

 Chubut (the tributary Teca) and in the River Genua ; they are 

 shales and red sandstones whose age it is difficult to fix, but it is 

 almost certain that they are not more modern than the Jurassic. 

 As they have not hitherto yielded fossils, nothing more precise can 

 be said. 



Cretaceous Strata. 



Upon these deposits comes a gigantic formation termed by 

 C. Ameghino the variegated sandstones (areniscas ahigarradas). 

 " They are composed chiefly of enormous banks of sand, generally 

 of fine grain, with beds of clay and intercalated conglomerates, 

 assuming all the colours and shades imaginable (bright red, purple, 

 ruddy, yellow, green, bronze, etc.)." It occupies the greater part 

 of the course of the River Chubut, a part of the region of the 

 Senguel, and in certain places extends even as far as the Atlantic 

 coast. Its thickness is to be reckoned in hundreds of metres, with 



1 C. Ameghino, " Exploraciones geologicas en la Patagonia" : Bol. Inst. Geograf. 

 Argent., toI. xi, 1890, pp. 1-46. 



