590 T, G. BONNEY.ON SOUTH-AMERICAN ROCKS, 
is probably omphacite; irregular grains of iron peroxide are also 
present, with quartz and a little apatite. Macroscopically this rock 
closely resembles, and microscopically is almost identical with, one — 
in my collection brought from Holsteinburg in South Greenland by 
Mr. P. H. Carpenter, and is nearly related to a specimen (locality 
unknown) given to me by Prof. Rupert Jones. The structure is 
rather abnormal ; but I think it must be an igneous rock, and thus 
is a variety of quartz-diorite. 
8. (Puerto de Tablas, p. 582.) A crystalline rock consisting chiefly 
of felspar with some quartz and a dark green mineral. The greater 
part of the slide is very characteristic microcline, with numerous wavy 
fibres of interbanded albite, as figured by Des Cloiseaux*. Minute 
enclosures are present, some opacite, some hornblende (?) microliths 
or opaque belonites, commonly arranged so as to cross at angles 
of about 60°, and roughly bisected by the albite bands. The green 
mineral is yery dark in colour, fairly strongly dichroic, and shows 
one very decided cleavage; it seems rather altered, and perhaps is 
partly replaced by a chloritic mineral; but, though not very cha- 
racteristic, it is probably hornblende. There are some grains of iron 
peroxide (some being hematite), and ferruginous stains are frequent ; 
also two or three grains of epidote and a little apatite (?). The rock 
appears to be of igneous origin, and so is a microclinic syenite. 
9. (Potosi Mines, p.586.) Much decomposed, but appear to have 
consisted of plagioclase felspar, augite, and ilmenite : the felspar isnow 
replaced by a saussuritic mineral; the augite rather decomposed, 
stained, and replaced by groups of microliths ; the ilmenite also much 
decomposed. The rock evidently has been a fine-grained dolerite, and 
for convenience may now be roughly grouped with the diabases. 
* Zeitschr. deutsch. geol. Gesellsch. (1875), vol. xxvii. p. 956. Not being at 
the time familiar with the aspect of microcline, I submitted this specimen to 
T. Davies, Hsq., F.G.S. (to whose extensive knowledge of minerals I have so 
often been indebted), and he pronounced it undoubtedly microcline, 
