5X2 Section by the Cumbre Pass, part n. 



from Professor E. Forbes's researches, that the sea at 

 greater depths than 600 feet becomes exceedingly 

 barren of organic beings, — a result quite in accordance 

 with what little I have seen of deep-sea soundings. 

 Hence, after this limestone with its shells was deposited, 

 the bottom of the sea where the main line of the 

 Cordillera now stands, must have subsided some thou- 

 sand feet to allow of the deposition of the superincum- 

 bent submarine strata. Without supposing a movement 

 of this kind, it would, moreover, be impossible to 

 understand the accumulation of the several lower strata 

 of coarse, well-rounded conglomerates, which it is 

 scarcely possible to believe were spread out in profoundly 

 deep water, and which, especially those containing 

 pebbles of quartz, could hai'dly have been rounded in 

 submarine craters and afterwards ejected from them, as 

 I believe to have been the case with much of the 

 porphyritic conglomerate formation. I may add that, 

 in Professor Forbes's opinion, the above-enumerated 

 species of Mollusca probably did not live at a much 

 greater depth than twenty fathoms, that is only 120 

 feet. 



To return to our section down the valley : standing 

 on the great N. by W. and S. by E. uniclinal axis of 

 the Puente del Inca, of which a section has just been 

 given, and looking north-east, great tabular masses of 

 the Gypseous formation [K K] could be seen in the 

 distance, very slightly inclined towards the east. Lower 

 down the valley, the mountains are almost exclusively 

 composed of porphyries, many of them of intrusive 

 origin and non-stratified, others stratified, but with the 

 stratification seldom distinguishable except in the 

 upper parts. Disregarding local disturbances, the beds 

 are either horizontal or inclined at a small angle east- 

 wards : hence, when standing on the plain of Uspallata 



