1884.] Microscopic Structure of Some Rocks from Ecuador. 429 



tain, in variable amount, cavities with bubbles, brown glass enclosures, 

 and other microliths, some being elongated prisms which may be apa- 

 tite ; but probably more than one mineral is present. There are also 

 some small fairly well-defined crystals of augite, but I have not suc- 

 ceeded in identifying any hypersthene. A remnant of a glassy base 

 appears to be present in the ground-mass, but it is so crowded with fel- 

 spar microliths, and with granules of iron peroxide and of augite, as 

 to be with difficulty distinguished. The felspar microliths are lath- 

 shaped ; they are a plagioclase ; but, as in the case of the larger crystals, 

 it is probable that more than one species is represented. The rock is 

 an augite-andesite, and its general aspect reminds me of some of the 

 porphyrites of the Cheviots {e.g., a hypersthene-andesite from Coquet, 

 near Windy Haugh). The third specimen, labelled Guagra-ialina 

 volcan, lado S.O. del Antisana, is a rather duller coloured, less 

 markedly porphyrinic rock than the last, having some minute vesicles. 

 The microscopic structure does not differ materially from that of the 

 last described. Possibly a little hypersthene is present, but this is 

 not conspicuous ; thus the rock is an augite-andesite, and all these 

 specimens may have come from different parts of the same flow or 

 from a closely related series of flows. 



From Quebrada de Urcucuy come two specimens of pitchstone. One, 

 labelled entre Tablarumi y Urcucuiloma, is a dark greenish-grey rock, 

 traversed by numerous cracks ; its fracture is very irregular, and it ex- 

 hibits the resinous lustre characteristic of pitchstone. A few minute 

 scattered crystals or grains of a glassy felspar are visible, and there 

 is a very faint indication of a fluidal structure. When examined 

 microscopically, the rock exhibits as a base a clear and colourless 

 glass. In this are scattered a large number of microlithic enclosures 

 together with some scattered crystals of larger size. The general 

 parallelism of the longer diameters of both of these, and the occa- 

 sional filamentous streaks of an aggregated grey dust render the 

 fluidal structure more conspicuous microscopically than macroscopi- 

 cally. The great majority of these microliths are little prisms or 

 columns, usually about 0*001 inch long, and commonly about one- 

 sixth of this in breadth. They are almost colourless, but appear to 

 have a slightly green tinge. I think it probable that, like the 

 belonites in the Arran pitchstones, to which they present some 

 resemblance, they are hornblende. Besides these, we find opacite, 

 with occasionally a fleck of brown mica or felspar crystals of small 

 size. The " dusty " bands are found to resolve themselves, when 

 viewed with a quarter-inch objective, into streams of microliths, like 

 to, but perhaps slightly smaller in size than, those described above. 

 Among the larger crystals are felspar : of this mineral orthoclase 

 and a plagioclase are present. Some of the crystals are rather broken 

 or rounded in outline, but others have well-defined external angles ; the 



