1884.] Microscopic Structure of some Rocks from Ecuador. 399 



taining crystals of the usual plagioclastic felspar, with a ferruginous 

 mica, grains of hematite or magnetite, and one or more pyroxenio 

 minerals, of which it is difficult to decide the species. The rock is an 

 andesite, but perhaps it is safest only to prefix the epithet micaceous. 



Sincholagua. 



" This is probably the tenth in altitude of the Ecuadorian Andes,* and 

 is situated about nortn -north-east of the cone of Cotopaxi at a distance 

 of 11 miles, and lies nearly east of the town of Machachi (whence it 

 is visible) at a distance of 13 miles. The culmination of this moun- 

 tain is a long ridge, rising at its highest point into a very sharp tooth, 

 to the summit of which we ascended, but were prevented whilst there 

 from taking any observations by the occurrence of a severe thunder- 

 storm. We made a precipitate retreat, remaining only long enough 

 to secure a couple of specimens." (E. W.) 



One of these, a portion of the actual summit, is a compact dark- 

 coloured rock, with a slightly rough fracture, containing numerous 

 small crystals of whitish felspar, generally not exceeding 'I inch in 

 the longer diameter. Under the microscope, the ground-mass is 

 seen to be a felted mass of minute elongated crystallites, probably 

 felspar, and of specks of opacite ; there is probably a residual 

 glassy base, but so numerous are the crystallites that it is by no 

 means easy to be sure. In this ground-mass are scattered larger 

 crystals of plagioclastic felspar similar to those already described, 

 augite, with probably some hypersthene (but it is not abundant), 

 and magnetite. The rock is thus an augite-andesite, probably hyper- 

 stheniferous. The other specimen taken in situ from the summit 

 peak, a little below the highest point, is so evidently a rock of similar 

 charactei% that I have thought it needless to have a specimen pre- 

 pared for microscopic examination. 



Cotocachi. 



M This mountain, though probably slightly inferior in elevation 

 to Sincholagua (and, consequently, only eleventh in order of the 

 Ecuadorian Andes), presents a much more imposing appearance than 

 its rank would lead one to expect. It is distant 45 miles in a direct 

 line from Quito, being a little east of north, and from that city forms 

 a prominent object. From the closer towns of Cotocachi, Otovalo, 

 and Ibarra, above whose level it rises 8000 to 9000 feet, at distances 

 of 8| to 15 miles, it wears a noble aspect. Reiss and Stiibel by the 

 mixed mere. bar. A method made its height 16,293 feet, and by 

 mere. bar. observations on its summit I found its elevation to be 

 16,301 feet. Its summit ridge has two principal points, the lower 

 being about 1 80 feet below the higher one. 



* Height, according to Eeiss and Stiibel, 16,365 feet. 



