1884.] Microscopic Structure of some Rocks from Ecuador, 431 



altitude is a little lower than the Hacienda of Antisana* (13,300 feet), 

 and so, if the indication of direction be correctly given, cannot be so 

 near as it to the summit. 



The specimen is a rather compact cream-coloured rock, at first 

 sight not unlike one of the South Tyrol dolomites, but slightly 

 vesicular in places, and spotted here and there with pale yellow 

 sulphur. It is evidently a volcanic rock from the vicinity of 

 fumaroles which deposit sulphur, and presents the usual aspect of a 

 trachyte which has been thus treated. Prom the appearance of the 

 rock, and a certain resemblance to one of those described, from 

 Quebrada de Urcucuy, I should think it probable it had once been a 

 pitchstone. 



A specimen is labelled Bel Nevado Pic 0, principio del Arenal ; the 

 il snowy peak" and the "sandy plain" of Antisana are localities un- 

 known to Mr. Whymper. The rock is subvitreous, dark in colour with 

 slightly redder streaks, and numerous scattered crystals of white 

 felspar, commonly not more than about O'l inch long, but now and 

 then twice or thrice as large. In the earliest stage of consolidation 

 are (1) plagioclase felspar (probably in part at least labradorite), some- 

 times irregular in external form, often crowded with glass cavities, 

 having fixed brownish bubbles, with microliths of augite (?), and with 

 opacite ; (2) augite ; (3) very characteristic crystals of hypersthene ; 

 (4) granules of iron oxide and a few scales of iron glance. The rock 

 has a glassy base, but this is crowded with lath-shaped felspar 

 microliths (plagioclase), and in most parts is rendered almost opaque 

 by dusty opacite and ferrite, the redder streaks being the more 

 transparent parts, in which a glass, now clear, now brown, may be 

 distinguished. The rock is a hyperstheniferous augite-andesite. 



The locality of the next specimen, labelled Cuspide del Achupallas 

 Lado 0. del Antisana, is also unknown to Mr. Whymper. The rock has 

 a deader lustre, and more scoriaceous aspect than the last described, 

 and contains greater crystals of whitish felspar, their diameter being 

 sometimes fully 0'3 inch. Under the microscope the larger of these 

 are seen to contain glass enclosures and other microliths, and are 

 probably labradorite ; the smaller, which are more lath-shaped, agree 

 better in their extinctions with oligoclase. There is a fair amount 

 of well characterised brown mica and of hornblende, both brown and 

 pale green varieties, with some granules of the latter or possibly of 

 augite, and some grains of iron oxide. There is a clear glassy base, 



* Beputed to be the highest farm in Ecuador. " It is situated on the western 

 slopes of Antisana, in a cheerless situation, without a tree in sight, and is enveloped 

 in fog the greater part of the year. The lower slopes of Antisana are of immense 

 length and very devoid of character on this side. The tipper 4,000 feet of the 

 western side of Antisana is almost entirely covered by glacier. The nearest to 

 the hacienda ends at an elevation of 15,295 feet." (E. W.) 



