T, G. BONNEY ON SOUTH-AMERICAN ROCKS. 589 
4, (Cunuri, p. 585.) A gneiss, though but little foliated: consists 
of quartz, felspar, mica, and epidote, with a little sphene, iron 
peroxide, and a few other microliths. The quartz is rather free 
from cavities and enclosures, what there are being very minute. 
The felspar is in rather irregular grains; orthoclase appears to pre- 
dominate, some showing cross-hatched structure. Several grains 
of felspar show twinning, by a series of rather faint bands of a 
darker tint. The mica appears to be biotite. One or two of the 
microliths may be apatite, others are short belonites of a very 
pale greenish, almost colourless, mineral (? hornblende); there are 
also a few specks of a secondary mineral, probably zeolitic. In 
general microscopic character this rock resembles a slide in my 
collection from a specimen collected at Godhaven, Disco Island, and 
its aspect is one which I have often noted in very ancient rocks. 
5. (Mano Piedra, p. 585.) <A basalt; small crystals of plagio- 
clase and clustered grains of augite, with iron peroxide, mostly 
ilmenite. I do not see any olivine. 
6. (Between Upata and Mano Piedra mountain, p. 584.) A ferri- 
ferous quartzite, consisting of subangular grains of rather clear 
quartz and of opaque iron peroxide, probably magnetite and 
ilmenite, in proportion of about two to one. Some of the smaller 
erains of peroxide are included in those of quartz ; the larger occupy 
interstices and are more irregular in form than the latter. A few 
of the little colourless microliths mentioned above (4) are present 
in the quartz, as well as some tufted trichites. A fair number of 
minute cavities are to be seen in the quartz ; they seem not to contain 
any fluid. 
7. (Upata, p. 584.) A crystalline rock consisting chiefly of rather 
irregularly formed grains of a felspar and a greenish mineral. The 
fomer is generally, if not always, a closely twinned plagioclase, and is 
remarkable for containing very numerous microliths ; these are pris- 
matic in form, resembling prisms of the monoclinic system, but gene- 
rally a little distorted, so that it is difficult to ascertain the exact shape 
of the section*; they are, on an average, about :01in. in length, and 
four or five times as long as broad. Asarule, they are arranged paral- 
lel to a twinning plane, and so probably lie in the brachydiagonal. 
The green mineral is rather intermediate in character between augite 
and hornblende; it is inirregular rounded grains; in texture it seems 
intermediate between fibrous or platy and granular, approaching in 
some respects to epidote; the cleavage is rather imperfect and not 
very characteristic, on the whole perhaps rather nearer that of 
augite, and is sometimes platy, rather like diallage. The mineral 
is very distinctly dichroic, sections roughly basal being pale brown, 
roughly prismatic pale olive-green ; the latter changes little in tint ; 
the former takes almost that of the other section, when the plane 
of polarization of the transmitted light is changed. This mineral 
occurs in a slide of eclogite from Eppenreuth in my collection, and 
* Similar microliths oceurin a felspar in no. 4, and in one in a gneiss from 
Ceylon in my collection. 
+ In two or three cases the grains seem to have imperfectly developed angles. 
Q.J.G.8. No. 140. 28 
