488 Section by the Portillo Pass. paet il 



ascends, passing by mountains composed of the gypseous 

 and associated beds, with their stratification greatly 

 disturbed and therefore not easily intelligible : hence 

 this part of the section has been left uncoloured. 

 Shortly before reaching the great Peuquenes ridge, the 

 lowest stratum visible [N] is a red sandstone or mud- 

 stone, capped by a vast thickness of black, compact, 

 calcareous, shaly rock [0], which has been thrown 

 into four lofty, though small ridges : looking northward, 

 the strata in these ridges are seen gradually to rise in 

 inclination, becoming in some distant pinnacles abso- 

 lutely vertical. 



The ridge of Peuquenes, which divides the waters 

 flowing into the Pacific and Atlantic Oceans, extends in 

 a nearly NNW. and SSE. line ; its strata dip eastward 

 at an angle of between 30° and 45°, but in the higher 

 peaks bending up and becoming almost vertical. Where 

 the road crosses this range, the height is 13,210 feet 

 above the sea-level, and I estimated the neighbouring 

 pinnacles at from 14,000 to 15,000 feet. The lowest 

 stratum visible in this ridge is a red stratified sandstone 

 [P] ; on it are superimposed two great masses [Q and 

 S] of black, hard, compact, even having a conch oidal 

 fracture, calcareous, more or less laminated shale, pass- 

 ing into limestone : this rock contains organic remains, 

 presently to be enumerated. The compacter varieties 

 fuse easily in a white glass ; and this I may add is a 

 very general character with all the sedimentary beds in 

 the Cordillera : although this rock when broken is 

 generally quite black, it everywhere weathers into an 

 ash-gray tint. Between these two great masses [Q and 

 S], a bed [R] of gypsum is interposed, about 300 feet 

 in thickness, and having the same characters as hereto- 

 fore described. I estimated the total thickness of these 

 three beds [Q, R, S] at nearly 3,000 feet ; and to this 



