1884.] Microscopic Structure of some Rocks from Ecuador. 121 



downwards. Some of these lie in positions to which they could 

 not have been brought by water, and they are doubtless volcanic 

 bombs. They are no doubt augite-andesites ; the actual rock having 

 a general likeness to that from the first camp, as well as to the 

 highest rock on Chimborazo (described below). 



From an elevation of from 15,000 to 16,000 feet he has brought a 

 number of pieces of a pumiceous scoria of rather glassy aspect. The 

 vesicles are of considerable size, and occupy more than half the 

 volume of each fragment. These specimens are of various shades of 

 dull grey in colour, being rather lighter than the last-named rocks, 

 and without the crystals of felspar. The rock is most probably also 

 an augite-andesite, but is evidently in a more glassy condition than 

 the others, although we must allow something to the greater tenuity 

 of the material. 



In fixing the tent at his first camp (about 15,100 feet) Mr. 

 Whymper (as mentioned above) dug through three layers of ash. At 

 the top was a miscellaneous accumulation of fragments, then came a 

 stratum of fine ash, about 7 inches deep, followed by a stratum of 

 slightly coarser material. 



The upper of these two strata consists of pumiceous lapilli of a pale 

 grey colour, often from an eighth to a quarter of an inch in diameter, 

 evidently closely allied in lithological character to the lighter scoria, 

 already described, from near this tent. There is with them a certain 

 admixture of finer dust, which chiefly consists of pulverised glass, 

 and has very probably to some extent been formed by attrition since 

 the material was collected. Mineral fragments are occasionally 

 distinguished. On the examination of a small portion mounted on a 

 slide, the mineral fragments appear more common. Most of these 

 are broken crystals, often with tolerably regular edges (defined no 

 doubt by cleavage planes), of a plagioclastic felspar, similar to that 

 described in the rock slides ; to several of these minute portions 

 of the scoriaceous rock are still adherent ; there are also one or two 

 fragments of a dull green augite, and two (in conjunction) of a 

 brownish-green mineral, which in form, extinction, and dichroism 

 agree with the mineral described in these papers as hypersthene. 

 Probably one or two more fragments occur, but they are less charac- 

 teristic ; the mineral is not common. The lower material is much 

 darker in colour, approaching nearer to the tint of the rock seen in 

 situ. There are occasional lapilli, ranging up to the size of a small pea, 

 but the most abundant are granules almost the size of the heads of 

 ordinary pins, or even a little smaller. On closer examination this 

 deposit is seen to consist of a dark scoria, not very vesicular, 

 together with occasional fragments of the usual minerals. These are 

 less recognisable under the microscope than in the case last described, 

 as they appear to be rather smaller and more embedded in a scoria- 



