1884.] Microscopic Structure of some Rocks from Ecuador. 137 



(E.) A dark-coloured rock with numerous specks of white felspar, 

 rather glassy-looking outside but with rough internal fracture. 



(D.) A rock similar to the last, but in shape a flattish slab rather 

 less than half an inch thick, reminding one* of the andesites and 

 phonolites used in Auvergne for roofing purposes. 



The specimens (A) were not very well suited for examination, and 

 were clearly only scoriaceous forms of andesites ; nearly allied to one 

 of the specimens in (B) ; of these I have not had slides made. 



(B.) The differences in these do not appear to be more than varietal ; 

 all are more or less vesicular, but in the one examined the cavities 

 are very small and not numerous ; the matrix is a dull purplish- 

 grey, weathering rather red externally, and minute white felspar 

 crystals are abundant. The microscopic structure differs so little 

 from those already described that it will be enough to say that the 

 ground-mass is rather opaque, and that there is present in it the 

 usual plagioclastic felspar, a fair amount of characteristic angite, and 

 a crystal or two of hypersthene. 



(C.) The less decomposed of the two specimens has been examined. 

 There are some varietal differences. The larger felspar crystals 

 (labradorite) are not quite so numerous as in the other, while crystal- 

 lites about '01 inch or rather less in longer diameter are very 

 numerous. There is a fair amount of well-characterised augite, with 

 grains of iron peroxide and opacite dust in a clear glassy base. 



(D) does not very materially differ except that the ground-mass of 

 the slide is rendered more opaque by the presence of opacite. 



(E) is a rock of similar character, except that the base is yet more 

 opaque. There is, however, one important distinction, that the 

 greater part of the pyroxenic constituent appears to be hypersthene, 

 and not augite. It is almost impossible to doubt the presence of an 

 orthorhombic mineral in this slide. 



On the summit of Corazon Mr. Whymper found two rock specimens, 

 evidently rudely dressed by hand, which will be described in his 

 forthcoming work on the Equatorial Andes. The material of these 

 bore a close resemblance to the rocks described above under the group 



From the above remarks it would appear that the crest of Corazon 

 consists of augite-andesites, which only exhibit slight varietal differ- 

 ences, except in the last case (E), where hypersthene becomes rather 

 abundant, apparently predominating over the ordinary pyroxene. It 

 is a remarkable fact that the exterior aspect of the rook had at once re- 

 minded me of those black, somewhat resinous-looking rocks formerly 

 variously called melaphyres, pitchstone-porphyrite, &c. which, of late 

 years, have been so frequently proved to contain hypersthene. 



