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



" Chimborazo appears to have been an extinct volcano for a long 

 period. The great size of its glaciers and complete effacement of its 

 crater ; the extent to which its rocks are decomposed and its ridges 

 shattered ; and the occurrence of lichens upon almost the highest 

 rocks which were collected, are all indications that it is long since it 

 was in a state of activity." (E. W.) 



Mr. Whymper brought back a large suite of specimens from Chim- 

 borazo ; from these I have selected eight for microscopic examination, 

 the others appearing to me to be either duplicates or Decomposed 

 specimens of the same or nearly identical rocks. From the locality 

 of the second camp I have examined two. The first was collected 

 from debris which had fallen from a cliff immediately above. Hence 

 though a loose specimen, it represents rock in situ at the locality ; the 

 rock is a black subvitreous lava with a few light coloured specks, pre- 

 senting a very close resemblance to that described above from the flank 

 of Cotopaxi ; one face of the specimen is scoriaceous. The microscopic 

 character does not materially differ. There are abundant crystals of 

 similar felspar (elongated forms being perhaps commoner) with 

 similar enclosures, also crystals of augite and hypersthene. One or 

 two grains, however, appear to me to be olivine. There is a base of 

 brownish glass, pretty full of microliths (chiefly of felspar), with 

 specks of ferrite and opacite, and perhaps a little pyroxene. 



The other specimen is of a type which, as will be seen below, is 

 common on Chimborazo, and presents resemblances to rock already 

 described, especially that from Pamascucho below Nina-urcu on 

 Pichincha ("Proceedings," No. 299, p. 225). This rock is a dullish 

 lavender-grey colour, with crystals of glassy felspars up to about 

 '1 inch long, and some minute blackish specks, which weather rather 

 a redder colour. Under the microscope the differences from the other 

 are not so great as perhaps might have been expected, the chief one 

 being that the base is a nearly colourless glass. I think it very 

 probable that a little sanidine is present among the felspars. The 

 rock then is only a variety of the hyperstheniferous augite-andesites. 



The specimen taken near the third camp of Chimborazo, 17,300 feet, 

 and representing, as described above, the rock which prevails through- 

 out the ridge by which the first ascent was made, from some distance 

 higher than the locality first mentioned, down to the second camp, 

 and even below, is a rock macroscopically related to the one last 

 described, but is a little redder in colour, more vesicular in structure, 

 and with slightly larger crystals of felspar (up to about ^th inch 

 diameter). So far as the base and its included microliths are con- 

 cerned, there is little to add to the preceding description, except that 

 a dusty ferrite is rather abundant, as the colour of the rock would lead 

 us to expect, the larger crystals of felspar do not materially differ from 

 those already described, hypersthene is abundant, undoubted augite 



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