chip. xiv. Uspallata Range. 533 



considering that in the Uspallata range some of the 

 beds, both low down and high up in the series, are 

 marked with vegetable impressions, showing the con- 

 tinued existence of neighbouring land ; — considering 

 the close general resemblance between the deposits of 

 this range and those of tertiary origin in several parts 

 of the continent ; — and lastly, even considering the 

 lesser height and outlying position of the Uspallata 

 range, — I conclude that the strata composing it are in 

 all probability of subsequent origin, and that they were 

 accumulated at a period when a deep sea studded with 

 submarine volcanos washed the eastern base of the 

 already partially elevated Cordillera. 



This conclusion is of much importance, for we have 

 'seen that in the Cordillera, during the deposition of 

 the Neocomian strata, the bed of the sea must have 

 subsided many thousand feet : we now learn that at a 

 later period an adjoining area first received a great 

 accumulation of strata, and was upheaved into land on 

 which coniferous trees grew, and that this area then 

 subsided several thousand feet to receive the superin- 

 cumbent submarine strata, afterwards being broken up, 

 denuded, and elevated in mass to its present height. I 

 am strengthened in this conclusion of there having 

 been two distinct, great periods of subsidence, by 

 reflecting on the thick mass of coarse stratified conglo- 

 merate in the valley of Tenuyan, between the Peuquenes 

 and Portillo lines ; for the accumulation of this mass 

 seems to me, as previously remarked, almost necessarily 

 to have required a prolonged subsidence ; and this 

 subsidence, from the pebbles in the conglomerate having 

 been to a great extent derived from the gypseous or 

 Neocomian strata of the Peuquenes line, we know must 

 have been quite distinct from, and subsequent to, that 

 sinking movement which probably accompanied the 



