430 



Prof. T. G. Bonney. On the 



[Mar. 13, 



latter are generally smaller and clearer, containing* a few belonites, 

 and but little else. The former are often ''dirty," containing glass 

 enclosures, cavities, and various microliths, as if belonging to an 

 earlier stage of consolidation. Besides these are several crystals of 

 brown mica and a few of hornblende, well denned, together with 

 scattered grains of magnetite. The cracks are marked by a pale 

 green staining, and there are no indications of a perlitic structure. 



The other specimen of pitchstone labelled entre Tablarumi y 

 Chacana is of nearly the same colour as the last described, but 

 contains many rounded whitish spots, roughly about T \ to ■§■ inch 

 diameter, which are seen on examination to be spherulites ; a portion 

 of the specimen is vesicular. The description given of the base of the 

 last specimen will serve for this, except that there is little indication 

 of a fluidal structure. There are a few scattered crystals of felspar, 

 brown mica, and hornblende. The spherulites are rather peculiar, 

 they have a rather irregular bluntly lobed outline, are nearly opaque, 

 but exhibit a faintly fibrous structure, something like that of groups 

 of blunt-pointed camel's hair brushes. So far as can be ascertained, 

 they consist of a brown glass traversed by belonites of a paler 

 mineral, and trichites of a darker one, but it is very difficult to 

 determine their exact structure. They generally enclose a small 

 crystal of hornblende or felspar, in one case both are present, but not 

 centrically disposed. Without chemical analysis one cannot decide 

 whether these two rocks are glassy forms of the rhyolites or of 

 the dacites, but I should be disposed to class them with the 

 latter.* 



A third specimen from Quebrada de Urcucuy, labelled in addition 

 Entre Tablarumi y Urcucuy, is a crumbling pale cream-coloured rock, 

 which, on closer examination, gives indications of having been glassy 

 and of a somewhat perlitic structure. This is confirmed by micro- 

 scopic examination, though the nature of the rock has prevented the 

 preparation of a good slide. It is evidently a decomposed perlitic 

 pitchstone, and very probably when in a fresh condition was nearly 

 related to the two others from this neighbourhood. f 



From the south western side is a specimen labelled Quebrada azufre 

 grande, S.O. Reiss and Stiibel, as Mr. Whymper informs me, 

 mention a " Quebrada azufre grande" giving a measurement Parte 

 inferior de la Loma al lado derecto de la Quebrada, Sfc, 4,040 metres 

 (13,255 feet). The name signifies " Great Sulphur" ravine. This 



* Since the reading of this paper Mr. J. J. H. Teall, F.G-.S., has kindly deter- 

 mined for me the specific gravity and silica percentage of the former of the two 

 pitchstones. S.g.=2'337; Si0 2 = 72*99, the loss on ignition being 115. These 

 determinations fully confirm the microscopic analysis. 



f A spherulitic pitchstone from Antisana is described by Vom Rath, " Yerh. Nat. 

 Ver. Preuss. Eheinl.," Folg. 4, Bd. 1 ; " Sitzungsb.," pp. 173, 174 (1874). 



i. 



