1884.] Microscopic Structure of some Rocks from Ecuador. 123 



No. I. (Tont.) A grey dust with rather darker specks. The grains 

 range from '02 inch in diameter downwards, a considerable pro- 

 portion varying between this and about '01 inch. They may be 

 thus distinguished : (A) rock fragments, (B) mineral fragments. 

 (A) These consist of (a) chips of colourless or nearly colourless glass, 

 sometimes almost clear, sometimes clouded with ferrite or opacite, and 

 containing microliths of felspar, &c. chips, in short, of glassy lavas, 

 similar to those described above and elsewhere in this paper; (6) 

 rough opaque, or nearly opaque, grains, rather more numerous and 

 larger in size, sometimes translucent at the edges, and including 

 microliths of felspar and augite ; these, when viewed with a dark 

 background, have a scoriaceous exterior, and are greyish, blackish, or 

 reddish-brown in colour ; they are evidently minute lapilli of an 

 andesitic lava. (B) Among these the following minerals may be 

 recognised : (a) felspar, showing occasionally plagioclastic twinning; 

 (6) more rare, augite and perhaps hypersthene. I notice fragments 

 both of glass and of minerals even among the finer dust, together 

 with black specks, probably magnetite. 



No. II. (Chimborazo.) Fine dust of a slightly paler and redder 

 colour than the last. The grains which make up this interesting 

 deposit, as indicated by a glance at the slides with the unaided eye, are, 

 as might be expected, decidedly smaller than those which characterise 

 No. I, a very few only attain to a diameter of '01 inch, and this is barely 

 exceeded. Fragments measuring from '003 to '004 inch are common, 

 and they vary from this size to the finest dust; the characteristic of the 

 deposit, so far as I can ascertain, being the presence of grains ranging 

 from about '001 to "003 inch. They consist, as before, of rock frag- 

 ments and mineral fragments. Among the former (A) the rough dark 

 lapilli are rare ; the majority being translucent and apparently smooth 

 externally. These are chips of glass, commonly of a pale brownish 

 colour, in which acicular microliths, probably of felspar, are frequent, 

 with specks of ferrite, and possibly a granule or two of a pyroxenic 

 mineral; vacuoles are certainly rare. (B) The mineral fragments are 

 felspar, as above, with a little augite, and there is one well-formed 

 hypersthene crystal '01 inch long, in which are enclosures of iron 

 peroxide, &c., and, I think, minute cavities. Fragments of felspar 

 and acicular crystallites are rather abundant among the finer dust. 



No. Ill (the dust from Ambato) does not materially differ from 

 No. II, except that perhaps the size of the fragments is rather more 

 variable. Although there is a large quantity of- very small chips 

 there is a slightly greater proportion of fragments about '01 in 

 diameter, and a considerable number of these are scoria, which is 

 almost opaque. In those that are transparent small vesicles, as 

 might be anticipated, are slightly more numerous than in the other 

 cases. 



