1884.] Microscopic Structure of some Hocks from Ecuador. 397 



Scattered in this, and belonging probably to an earlier stage of con- 

 solidation, are the following minerals : (a) plagioclastic felspar, often 

 about '02 to *04 inch diameter, probably, as usual, labradorite. Many 

 of these have a broken and corroded aspect, this being more than 

 usually conspicuous ; (ft) magnetite ; (c) grains of a pyroxenic 

 mineral. It is rather difficult to decide upon the species of the last ; 

 some of the grains are prismatic in form, but others have a more or 

 less rounded outline. The cleavage seems imperfect. The mineral is 

 clear, colourless, and of a rather frosted texture. The imperfection 

 of the external form and of the cleavage makes it difficult to obtain 

 clear indications of the crystalline system. Brilliant colours are given 

 with crossing Nicols. It is not impossible the mineral may be mono- 

 clinic, still some results suggest the probability that it is ortho- 

 rhombic. If an augite, it is rather abnormal, but we must be prepai'ed 

 for the presence of some other magnesian silicate, such as olivine or 

 (more probably) enstatite. Be this as it may, the rock itself is a 

 member of the augite-andesite group. 



The rock D, from the north side of Altar, has a different aspect, 

 being (like some already described) a reddish-grey trachyte, studded 

 with crystals of rather glassy white felspar, roughly about '1 inch 

 diameter, and containing some minute vesicles. It has not yielded a 

 very good slide. The ground-mass appears to be a clear glass, with 

 numerous lath-like crystallites of felspar, but is so thickly crowded 

 with ferrite and opacite, especially the former, as to be all but opaque 

 except in the thinnest part of the section. In this ground-mass are 

 scattered crystals of the usual plagioclastic felspar, and a pyroxenic 

 mineral ; some of this is monoclinic augite, but I think a few grains 

 are certainly orthorhombic. Thus the rock is an augite-andesite, 

 probably containing some hypersthene. 



Illiniza. 



" This mountain is probably the seventh in rank of the Ecuadorian 

 Andes, and is the most southern of the four which lie (approximately) 

 in a line.* It bears exactly due west of Cotopaxi, at a distance of 

 about 20 miles. The road to Quito passes over the ridge which in a 

 manner connects the two mountains. According to Messrs. Beiss 

 and Stiibel its height is 17,405 feet. On February 9, 1880, I reached 

 the height of 17,023 feet on the south side, upon June 9, 1880,- the 

 height of 16,922 feet on its north side, and in the middle of May, my 

 two assistants, Louis and J. A. Carrel, gained the summit from the 

 north. 



" The summit of Illiniza proper (there is a secondary peak on the 

 northern side which is termed Little Illiniza) is exceedingly sharp, 



* The four (commencing from the north) are Pichincha, Atacatzo, Corazon, 

 Illiniza. 



2 D 2 



