590 Summary of the Structure pabt n. 



was with the porphyritic conglomerate. Hence in 

 following thus far the geological history of the Cor- 

 dillera, it may be inferred that the bed of a deep and 

 open, or nearly open, ocean was filled up by porphyritic 

 eruptions, aided probably by some general and some 

 local elevations, so that comparatively shallow level at 

 which the cretaceo-oolitic shells first lived. At this 

 period, the submarine craters yielded at intervals a 

 prodigious supply of gypsum and other mineral exhala- 

 tions, and occasionally, in certain places poured forth 

 lavas, chiefly of a feldspathic nature : at this period, 

 islands clothed with fir-trees and composed of porphy- 

 ries, primary rocks, and the lower gypseous strata had 

 already been locally upheaved, and exposed to the 

 action of the waves ; — the general movement, however, 

 at this time having been over a very wide area, one ot 

 slow subsidence, prolonged till the bed of the sea sank 

 several thousand feet. 



In Central Chile, after the deposition of a great 

 thickness of the gypseous strata, and after their up- 

 heaval, by which the Cumbre and adjoining ranges were 

 formed, a vast pile of tufaceous matter and submarine 

 lava was accumulated, where the Uspallata chain now 

 stands ; also after the deposition and upheaval of the 

 equivalent gypseous strata of the Peuquenes range, the 

 great thick mass of conglomerate in the valley of 

 Tenuyan was accumulated : during the deposition of 

 the Uspallata strata, we know absolutely, from the 

 buried vertical trees, that there was a subsidence of 

 some thousand feet ; and we may infer from the nature 

 of the conglomerate in the valley of Tenuyan, that a 

 similar and perhaps contemporaneous movement there 

 took place. We have, then, evidence of a second great 

 period of subsidence ; and, as in the case of the sub- 

 sidence which accompanied the accumulation of the 



