SURFACE SOIL. SIX FEET DEEP 



FINER BASALTIC ASH, TWELVE INCHES 



104 TRAVELS A3I0NGST THE GREAT ANDES. chap. t. 



and may properly be termed an impalpable powder. It is found 

 to be principally made up of felspar and hornblende^ with some 

 pumice and a small admixture of mica and magnetic particles.^ 



Underneath it comes a 

 dark and comparatively 

 coarse basaltic ( ?) ash, two 

 TRACHYTic DUST, TEN INCHES fcct aud a lialf tlilck ; and 



this is succeeded by a finer 



, , .-, ,,,.;_ ash of the same nature, 



one foot deep. In these 

 two strata, three feet and 

 a half thick, pumice occurs 



VOLCANIC DUST, DEPTH UNKNOWN OUly lU SUiali qUaUtltlCS. 



Underneath them, extend- 

 ing how deep I do not 

 know, there is a fine and brilliantly white dust of a totally dis- 

 tinct character from all above it. This mainly consists of pumice, 

 and closely approximates in its constitution to the coarser ash 

 which was found at Ambato (see pp. 94-5).^ These four beds are 



' Prof. T. G. Bonney and Miss Raisin say: "It consists largely of mineral 

 fragments, which are often of broken crystalline form. The coarser vary from 

 •05 to -15 mm. in length, the finer may average from -01 to -02 mm. Pumice 

 is present, some of it enclosing fairly large crystals. The minerals in the ash are 

 chiefly felspar (some being contained within the pumice) and green pyroxene, so far 

 as could be ascertained, hornblende. A few largish chips of brown mica occur, 

 and some black opaque grains, probably an iron oxide. The finest dust seems to 

 consist chiefly of felspar and of pumice." 



2 Almost the sole point of difference between them is that the Ambato deposit 

 contains a considerable percentage of rocky fragments. This is better seen in 

 bulk than in microscopic samples. 



"The lowest stratum at Machachi consists mainly of clear, colourless vesicular 

 pumice, which includes greenish mica, some in minute hexagonal plates. This 

 ash is very like that from Ambato" (described on p. 94), "but is rather clearer, 

 having fewer of the microlithic aggregations, and it contains more numerous grains 

 of a clear felspar, mostly in angular chips. Brown or greenish spheroids occur, 

 which are probably similar to those of Ambato ; they are very regular in form, 

 sometmies shewing rounded holes or granular structure, and they rather mimic 

 the appearance of casts of organisms."— Prof. T. G. Bonney and Miss C. A. Raisin. 



