CHAP. XVIII. DESCRIPTIONS OF VOLCANIC DUSTS. 329 



ejected during intermittent discharges of steam, and, presumably, 

 was torn off by the violence of the blasts. It is reasonable to find 

 that there is a considerable difference in the weight and dimensions 

 of the particles of these two dusts. The larger and heavier atoms 

 naturally settle soonest, and the smaller and lighter ones travel 

 farthest. Upon several occasions I have endeavoured to count the 

 number of particles in a grain of the Chimborazo deposit, and have 

 found that the smaller ones do not weigh so much as 2iTo^Tro" P^^^ 

 of a grain, and that the finer atoms are lighter still. ^ 



1 Professor Bonney has examined the two dusts microscopically, and has favoured 

 me with the following descriptions. No. I. {Dust toJiichfell on the tent at the summit 

 of Cotopaxi, Feb. 18-19, 1880). "The grains range from '02 inch in diameter 

 downwards, a considerable proportion 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. (6) rough opaque, or nearly opaque 

 grains, sometimes translucent at the edges, and including microliths of felspar and 

 augite ; these, when viewed Avith 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 folloMing minerals may be recog- 

 nized : — (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. (Dust ivhich was ejected by Cotopaxi upon July 3, 1880, and fell upon 

 the tent at the fifth camp on Chimborazo, distant sixty four miles). " 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 'OOS 

 inch. They consist, as before, of rock fragments 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. . . (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." 



2u 



