chap. xiv. the Portillo Range. 501 



as from the evidence they afford by their inclination 

 taken conjointly with their thickness and compactness, 

 that after the great range had assumed its present 

 general outline, it continued to rise as an axis of eleva- 

 tion. The plains extending from the base of the 

 Cordillera to the Atlantic show that the continent has 

 been upraised in mass to a height of 3,500 feet, and 

 probably to a much greater height, for the smooth 

 shingle-covered margin of the Pampas is prolonged in a 

 gentle unbroken slope far up many of the great valleys. 

 Nor let it be assumed that the Peuquenes and Portillo 

 ranges have undergone only movements of elevation; 

 for we shall hereafter see, that the bottom of the sea 

 subsided several thousand feet during the deposition of 

 strata, occupying the same relative place in the Cor- 

 dillera, with those of the Peuquenes ridge; moreover, 

 we shall see from the unequivocal evidence of buried 

 upright trees, that at a somewhat later period, during 

 the formation of the Uspallata chain, which corresponds 

 geographically with that of the Portillo, there was 

 another subsidence of many thousand feet : here, in- 

 deed, in the valley of Tenuyan, the accumulation of the 

 coarse stratified conglomerate to a thickness of 1,500 

 or 2,000 feet, offers strong presumptive evidence of 

 subsidence ; for all existing analogies lead to the belief 

 that large pebbles can be transported only in shallow 

 water, liable to be affected by currents and movements 

 of undulation — and if so, the shallow bed of the sea on 

 which the pebbles were first deposited must necessarily 

 have sunk to allow of the accumulation of the super- 

 incumbent strata. What a history of changes of level, 

 and of wear and tear, all since the age of the latter 

 Secondary formations of Europe, does the structure of 

 this one great mountain-chain reveal ! 



