IxXVi PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



crystals of hornblende, with mica, chlorite, epidote and quartz. Where 

 the mica and quartz are abundant, the rock cannot be distinguished 

 externally from granite. A brick-red granite composed of orthitic or 

 potash-felspar occurs in the Portillo range, which Mr. Darwin is 

 inclined to think is of newer formation than the rock of which albite 

 is the chief constituent. 



After ascending the Peuquenes Pass to a height of 7000 feet, a vast 

 formation of gypseous strata begins to appear. It is partly composed 

 of beds of snow-white hard gypsum with a saccharoid fracture, and 

 partly of a pale brown argillaceous gypsum, highly inclined, and 

 conformable in stratification with those of the porphyritic conglo- 

 merate on which they repose. The gypseous beds are covered by a 

 red sandstone, seen in some places to be 1 000 feet thick ; this again 

 is covered by gypseous beds of equal thickness, and these in their 

 turn are surmounted by a repetition of the red sandstone. Above 

 the latter rock there occurs a black, compact, calcareous shaly rock 

 of vast thickness. From these last strata Mr. Darwin collected two 

 Ammonites, a Grj^phsea, a Natica, a Cyprina, a Rostellaria, and a 

 Terebratula, which having been examined by M. Alcide d'Orbigny, 

 were considered by him to belong to the Neocomian stage of the 

 Cretaceous system. Fossils collected in another part of the same 

 formation were pronounced by M. von Buch to indicate a formation 

 intermediate between the limestone of the Jura and the chalk, ana- 

 logous with the uppermost Jurassic beds forming the plains of S-wit- 

 zerland. The fossils collected by Mr. Darwin were imbedded in the 

 rock at the height of 13,200 feet, and the same beds are prolonged 

 upwards to at least from 14,000 to 15,000 feet above the level of the 

 sea. These strata have been greatly disturbed, dipping both west and 

 east, the remnants of an anticlinal ridge, and they also dip towards 

 the centre of the range. 



A similar series of beds occurs on the eastern flank of the Cumbre 

 range, but associated with numerous alternations of porphyritic and 

 felspathic rocks, with all the characters of submarine contempora- 

 neous lavas. The flanks of the mountain are here quite bare and 

 steep, afl'ording a section of a series of strata whose united thickness 

 must be nearly 6000 feet : from the lowest to the uppermost bed 

 of gypsum, it cannot be less than 2000 feet. There is however 

 this important difference between the Cumbre series and that of the 

 Peuquenes, that the limestone, containing the same fossils as that of 

 the Peuquenes which lies there near the top of the series, at the Cumbre 

 lies at the very base of the formation, just above the porphyritic 

 conglomerate — that is, several thousand feet lower in the series ; and 

 it forms a stratum 80 feet thick. In the opinion of M. von Buch 

 and M. d'Orbigny, the two formations belong to the same age. Pro- 

 fessor Edward Forbes has likewise a strong impression that they in- 

 dicate the cretaceous period, and probably an early epoch in it ; and 

 Mr. Darwin himself is of opinion, that probably the gypseous and 

 associated beds in all the sections belong to the same great formation, 

 and he has denominated it cretaceo-oolite. Similar strata have been 

 observed farther north in Southern Peru by Mr. Darwin and M. 



