1884.] Microscopic Structure of Some Rocks from Ecuador. 427 



point, may possibly fill and hide a crater. The diameter of the nearly 

 level area which forms the summit is about equal to that of the lip 

 of the crater of Cotopaxi. It is also certain that there is no open 

 crater on any part of the western slopes of the mountain." Of the 

 remainder Mr. Whymper says : " I speak with less confidence of the 

 northern and southern sides, as I have not seen completely round 

 them, and of the eastern side I can only speak of the parts not more 

 than 1,000 feet below the summit. Upon my first and unsuccessful 

 attempt to ascend the mountain we were stopped for a considerable 

 time (by the difficulties encountered) at a height of somewhat more 

 than 17,000 feet above the sea, and, whilst waiting, we noticed several 

 puffs of strongly sulphurous vapour. We did not, however, observe 

 either upon the summit, or when viewing it from a distance, anything 

 of the nature of an eruption, or learn from the persons living in the 

 vicinity of the mountain that any eruption had occurred to their 

 knowledge." 



Mr. Whymper's collection from Antisana consists, as in the case of 

 Pichincha, of a series of ten specimens obtained in Quito, and 

 fourteen collected by himself. Four of the latter shall be described 

 first, as they come from the lower part of the mountain, from a spot 

 called Antisanilla, which however is 12,340 feet above the sea. Here 

 a hacienda abuts against a great lava stream which has descended from 

 the mountain, and is the one most familiar to the natives of the dis- 

 trict. Mr. Whymper remarks that it was the only large stream of lava 

 which he observed on the western side of the mountain running on 

 towards the west : " Its full extent 1 do not know, owing to mist. We 

 coasted its southern side for 5 or 6 miles on the way to the Hacienda 

 of Antisana (13,300 feet), and the small Hacienda of Antisanilla, an 

 appendage of the large establishment, is built by the side of the lava 

 stream, which was by various persons several times termed in my 

 hearing the lava of Antisanilla. The surface of the stream was 

 extremely rugged and well-nigh inaccessible." 



From this lava Mr. Whymper collected specimens : the one selected 

 for microscopic examination is a black sub- vitreous rock, containing 

 small crystals of white felspar, whose diameter is commonly not more 

 than 0*125 inch. The general aspect of the specimen shows it to be 

 one of the darker varieties of andesite, a member of the group of rocks 

 that have been variously named melaphyre, pitchstone-porphyrite, &c. 



The crystals belonging to the earlier stages of consolidation which 

 are included in the slide are rather small, no one of the felspars ex- 

 ceeding 0*1 inch, and only one or two approaching this size. They are 

 plagioclastic, but as the majority have broken away in grinding the 

 slide, one cannot venture to give a more definite name. The crystals 

 of pyroxenic minerals are yet smaller ; most of the latter occurring 

 either in small scattered crystals about 0*006 inch long, of rather 



