1884.] Microscopic Structure of some Rocks from Ecuador. 405 



a middling grain, with light reddish felspar, a small quantity of 

 blackish-green mica, and a great deal of reddish-grey quartz. Again, 

 "further to the south and a little to the east of the road leading from 

 Riobainba Nuevo (9483), . . . mica slates and gneiss everywhere 

 make their appearance towards the foot of the colossal Altar de los 

 Collanes, the Currillan, and the Parama del Antillo." From these and 

 other instances, he concludes that the great volcanic mountains rest 

 on a foundation of schists or gneisses. 



Tonalite or Quartz-diorite. 



This specimen was taken by Mr. Whymper from a boulder in the 

 bed of the S. Jorge River, at the place where the ordinary track 

 from Bodegas to Quito crosses the stream a little above the village of 

 Manapamba, about 1350 feet above the sea, and at the commencement 

 of the ascent of the Pacific slopes of the outer range of the Andes. 

 This was the only place where Mr. Whymper found any rock resem- 

 bling a granite, and he nowhere saw it in situ. It is macroscopically 

 a fairly coarse granite-like rock in which a whitish felspar and dark 

 green hornblende are the more conspicuous minerals. Under the 

 microscope it is seen to consist of the following minerals : Felspar, 

 which in the great majority of cases exhibits either zonal banding 

 or polysynthetic twinning, and may, I think, be safely regarded as 

 principally oligoclase; hornblende with well characterised cleavage 

 and external angles ; a brown mica, sometimes almost opaque, 

 frequently in aggregated flakes ; quartz in granules, as in a granite, 

 containing many fluid-cavities from '0001 to '0003 inch, in which 

 commonly are bubbles occupying about ^ or of their volume, 

 magnetite, or perhaps in some cases ilmenite, possibly a little apatite. 

 The hornblende appears to have crystallised nearly simultaneously with 

 the felspar, but perhaps a little earlier, the quartz last of all. The 

 rock is in moderately good preservation. It has macroscopically and 

 microscopically a considerable resemblance to the typical tonalite of 

 the Adamello district. 



Granite. 



There is a small specimen of this rock of which Mr. Whymper 

 writes : 



" At the town of Bodegas I found a specimen of granite in the 

 possession of an old English resident in Ecuador, Mr. Wilson, who 

 gave me a fragment from it. He told me that he had collected it 

 in situ on the track leading from the town of Riobamba (9000 feet) to 

 the village of Banos (5905 feet, R. & S.), but I could not learn at what 

 altitude it was taken. The locality is in latitude a little north of 

 the spot at which the last described specimen was collected, and is 

 on the opposite side of the Andes, that is to say, on the Amazonian 



