THE SEMI-MOUNTAINOUS REGION OF THE EASTERN PLAINS. 235 



the very midst of the Andes, would seem to preclude the possibility of 

 this having been a mountainous region during the deposition of the 

 materials from which these lignites were formed. For, while a mountain- 

 ous country may and often does possess all the characteristics essential 

 to the growth of a luxuriant vegetation and for the accumulation of small 

 bodies of peat, which later might be transformed into coal, it does not 

 present conditions compatible with the deposition of extensive coal-fields, 

 such as those of this region are believed to have been. 



From the facts set forth in the preceding paragraph I have arrived at 

 the following conclusions : ( i ) That, dnring the deposition of the Pata- 

 gonian beds, the soidhern Andean region was occupied by a shallow sea, 

 which was, at most, interrupted by only a few small and rocky islets. (2) 

 That, during and after the deposition of the Santa Cruzian beds in Miocene 

 and early Pliocene times, this region was a broad, but little elevated plain, 

 with numerous and extensive marshes, but no real mountains then existed 

 where now are the Andes. (3) That, during the deposition of the Cape 

 Fairweather beds in late Pliocene times, there was another submergence, 

 with conditions very similar to those which attetided the previotts period of 

 submergence. (4) That, with the close of the Pliocene, the final emergence 

 of this region began and that the principal elevation, if not the actual birth, 

 of the southern Andes dates from this period. 



The Semi-mountainous Region of the Eastern Plains. — While the 

 Andes are in reality the only true mountains that lie within the limits of 

 our region, yet there is a considerable area to the eastward of these 

 mountains that displays many features characteristic of a truly moun- 

 tainous region. I refer to the deeply dissected and usually basalt-cov- 

 ered region to the eastward of Lakes Pueyrredon and Buenos Aires. 

 This region is at present imperfectly known, but during some six weeks 

 spent in exploring it in the autumn of 1898, I was able to learn much 

 concerning its present nature and probable past history. It consists 

 of a complicated series of porphyries and other crystalline rocks, in 

 places much disturbed and everywhere deeply dissected, both by recent 

 and ancient valleys of erosion. Over the deeply excavated surface of 

 these older crystalline materials there has been deposited a series of sedi- 

 mentary rocks of both Mesozoic and Tertiary age. Remnants of these 

 later deposits now appear at frequent intervals along the sides of the more 

 recently eroded canons, where they are to be seen filling the older valleys, 



