chap. xiv. The Portillo Range, 499 



Concluding Remarks on the Portillo Range. — When 

 on the Pampas and looking southward, and whilst travel- 

 ling northward, I could see for very many leagues the 

 red granite and dark mica- schist forming the crest and 

 eastern flank of the Portillo line. This great range, 

 according to Dr. Gillies, can be traced with little inter- 

 ruption for 140 miles southward to the R. Diamante, 

 where it unites with the western ranges : northward, 

 according to this same author, it terminates where the 

 R. Mendoza debouches from the mountains ; but a 

 little farther north in the eastern part of the Cumbre 

 section, there are, as we shall hereafter see, some 

 mountain-masses of a brick-red porphyry, the last in- 

 jected amidst many other porphyries, and having so 

 close an analogy with the coarse red granite of the Por- 

 tillo line, that I am tempted to believe that they belong 

 to the same axis of injection ; if so, the Portillo line is 

 at least 200 miles in length. Its height, even in the 

 lowest gap on the road, is 14,365 feet, and some of the 

 pinnacles apparently attain an elevation of about 

 16,000 feet above the sea. The geological history of 

 this grand chain appears to me eminently interesting. 

 We may safely conclude, that at a former period the 

 valley of Tenuyan existed as an arm of the sea, about 

 twenty miles in width, bordered on one hand by a ridge 

 or chain of islets of the black calcareous shales and 

 purple sandstones of the Gypseous formation ; and on 

 the other hand, by a ridge or chain of islets composed 

 of mica-slate, white granite, and perhaps to a partial 

 extent of red granite. These two chains, whilst thus 

 bordering the old sea-channel, must have been exposed 

 for a vast lapse of time to alluvial and littoral action, 

 during which the rocks were shattered, the fragments 

 rounded, and the strata of conglomerate accumulated 

 to a thickness of at least 1,500 or 2,000 feet. The 



