242 cosmos: 



of Sorata (21,292 feet) and Illimani (21,155 feet) consist 

 of graywacke schists, which are penetrated by porphyritic 

 masses,* in which (as a proof of this penetration) fragments 

 of schist are inclosed. In the eastern Cordillera of Quito, 

 south of the parallel of 1° 35', the high summits (Condoras- 

 to, Cuvillan, and the Collanes) lying opposite to the tra- 

 chytes, and also entering the region of perpetual snow, are 

 also mica-slate and fire-stone. According to our present 

 knowledge of the mineralogical nature of the most elevated 

 parts of the Himalaya, which we owe to the meritorious la- 

 bors of B. H. Hodgson, Jacquemont, Joseph Dalton Hooker, 

 Thomson, and Henry Strachey, the primary rocks, as they 

 were formerly called, granite, gneiss, and mica-slate, appear 

 to be visible here also, although there are no trachytic for- 

 mations. In Bolivia, Pentland has found fossil shells in the 

 Silurian schists on the Nevado de Antacaua, 17,482 feet 

 above the sea, between La Paz and Potosi. The enormous 

 height to which, from the testimony of the fossils collected 

 by Abich from Daghestan, and by myself from the Peruvian 

 Cordilleras (between Guambos and Montan), the chalk for- 

 mation is elevated, reminds us very vividly that non-volcan- 

 ic sedimentary strata, full of organic remains, and not to be 

 confounded with volcanic tufaceous strata, show themselves 

 in places where for a long distance around melaphyres, 

 trachytes, dolerites, and other pyroxenic rocks, which we 

 regard as the seat of the upheaving, urging forces, remain 

 concealed in the depths. In what immeasurable tracts of 

 the Cordilleras and the districts bordering them upon the 

 east is no trace of any granitic formation visible ! 



The frequency of the eruptions of a volcano appearing to 

 depend, as I have already repeatedly observed, upon multifa- 



* These penetrating porphyritic masses show themselves in peculiar 

 vastness near the Illimani, in Cenipampa (15,919 feet) and Totora- 

 pampa (13,709 feet) ; and a quartzose porphyry containing mica, and 

 inclosing garnets, and at the same time angular fragments of silicious 

 schist, forms the superior dome of the celebrated argentiferous Cerro 

 de Potosi (Pentland in MSS. of 1832). The Illimani, which Pent- 

 land estimated first at 7315 (23,973 feet), and afterward at 6415 

 (21,139 feet) metres, has also been, since 1817, the object of a care- 

 ful measurement by the engineer Pissis, who, on the occasion of his 

 great trigonometrical survey of the Llanura de Bolivia, found the Illi- 

 mani to be on the average 6509 metres (21,349 feet) in height, by 

 three triangles between Calamarca and La Paz : this onlv ditfers 

 about 64 metres (210 feet) from Pentland's last determination. See 

 Investigaciones Sobre la Altitud de los Andes, in the Anales de Chile, 

 1852, p. 217 and 22l. 



