532 Cone hiding Remarks on the part ii. 



veins and concretionary arrangement, and those grit 

 and mud stones, and singular semi-porcellanic rocks, 

 so abundant in the Uspallata range. The submarine 

 lavas, also, differ considerably ; the feldspathic streams 

 of the Cordillera, contain much mica, which is absent in 

 those of the Uspallata range : in this latter range we 

 have seen on how grand a scale, basaltic lava has been 

 poured forth, of which there is not a trace in the 

 Cordillera. This dissimilarity is the more striking, 

 considering that these two parallel chains are separated 

 by a plain only between ten and fifteen miles in width ; 

 and that the Uspallata lavas, as well as no doubt the 

 alternating tufaceous beds, have proceeded from the 

 west, from points apparently between the two ranges. 

 To imagine that these two piles of strata were con- 

 temporaneously deposited in two closely adjoining, very 

 deep, submarine areas, separated from each other by 

 a lofty ridge, where a plain now extends, would be a 

 gratuitous hypothesis. And had they been contempo- 

 raneously deposited, without any such dividing ridge, 

 surely some of the gypseous and other sedimentary 

 matter forming such immensely thick masses in the 

 Cordillera, would have extended this short distance 

 eastwards ; and surely some of the Uspallata tuffs and 

 basalts also accumulated to so great a thickness, would 

 have extended a little westward. Hence I conclude, 

 that it is far more probable that these two series are 

 not contemporaneous ; but that the strata of one of the 

 chains were deposited, and even the chain itself uplifted, 

 before the formation of the other : — which chain, then, 

 is the oldest ? Considering that in the Uspallata range 

 the lowest strata on the western flank lie unconforma- 

 bly on the clay-slate, as probably is the case with those 

 on the eastern flank, whereas in the Cordillera all the 

 overlying strata lie conformably on this formation : — 



