chap. xiv. Plain of Uspallata. 5 1 5 



am strongly urged to believe (as formerly remarked) 

 that the grand mountain-masses composed of this brick- 

 red porphyry belong to the same axis of injection with 

 the granite of the Portillo line : if so, the injection of 

 this porphyry probably took place, as long subsequently 

 to the several axes of elevation in the gypseous forma- 

 tion near the Cumbre, as the injection of the Portillo 

 granite has been shown to have been subsequent to the 

 elevation of the gypseous strata composing the Peu- 

 quenes range ; and this interval, we' have seen, must 

 have been a very long one. 



The Plain of Uspallata has been briefly described 

 in Chap. X. ; it resembles the basin-plains of Chile ; 

 it is ten or fifteen miles wide, and is said to extend for 

 180- miles northward; its surface is nearly 6,000 feet 

 above the sea ; it is composed, to a thickness of some 

 hundred feet of loosely aggregated, stratified shingle, 

 which is prolonged with a gently sloping surface up the 

 valleys in the mountains on both sides. One section 

 in this plain [Z] is interesting, from the unusual 1 cir- 

 cumstance of alternating layers of almost loose red and 

 white sand with lines of pebbles (from the size of a nut 

 to that of an apple), and beds of gravel, being inclined 

 at an angle of 45°, and in some spots even at a higher 

 angle. These beds are dislocated by small faults : and 

 are capped by a thick mass of horizontally stratified 

 gravel, evidently of subaqueous origin. Having been 

 accustomed to observe the irregularities of beds accu- 

 mulated under currents, I feel sure that the inclination 

 here has not been thus produced. The pebbles consist 

 chiefly of the brick-red porphyry just described and of 

 white granite, both probably derived from the ranges 

 to the west, and of altered clay-slate and of certain 



1 I find that Mr. Smith, of Jordanhill, has described (' Edinburgh 

 New Phil. Journ.' vol. xxv. p. 392) beds of sand and gravel, near Edin- 

 burgh, tilted at an angle of 60°, and dislocated by miniature faults. 



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