588 T. G. BONNEY ON SOUTH-AMERICAN ROCKS. 
with the observations made by myself in the field, and also with the 
analyses previously alluded to. 
It is not improbable that many minor details are omitted in my 
section, considering the difficulties under which the observations were 
made, my sole object being to explain the general geological features 
of a comparatively little-known district. 
Note on some Rocks from Sourm AMERICA. 
By the Rey. Prof. T. G. Bonnny, M.A., F.RB.S., Sec. G.S. 
1. (Near Potosi, p.586.) This rock has been much altered. The 
following constituents are present :—(a) crystalline grains composed 
of earthy-looking granules and minute, rather fibrous microliths, 
showing light colour with the two Nicols, being one of the decompo- 
sition pseudomorphs after felspar, often called saussurite ; (b) augite 
and diallage, more or less converted into hornblende and a chloritic 
mineral; (¢)iron peroxide, probably ilmenite ; (d)a little apatite. The 
general aspect of the saussuritic grains suggests that the felspar has 
been plagioclastic. Of the pyroxenic mineral the greater part is 
altered, being converted into hornblende with strong dichroism and 
very characteristic cleavage, associated with which are smaller more 
fibrous forms, some of which are probably varieties of hornblende, 
others rather more resemble chlorite. Portions of the diallage and 
augite remain unchanged in many of the grains; both minerals ap- 
pear to be present. One or two serpentinous-looking aggregates 
may possibly replace olivine. We may venture to say that the rock 
has once been a gabbro, in which the pyroxenic constituent has 
been almost wholly converted into hornblende, as has not seldom 
happened*. 
2. (Quacipati, p. 585.) The ground-mass of the rock consists 
chiefly of minute grains of quartz, looking as if fused together. In 
this are scattered many small grains of epidote and scales of mica. 
Of the latter there appear to be two kinds—one having its darkest 
tint an olive-green, rather strongly dichroic; the other nearly colour- 
less, and showing brilliant tints with the two Nicols, rather like para- 
gonite ; indeed these two types of mica are common in the granitoid 
eneiss of the St. Gothard. In this ground-mass are several crystal- 
line grains, not very clearly defined in outline, and containing the 
epidote and mica-granules. One or two rather resemble kyanite ; 
but, as far as I can make out, they are only felspar, and two or three 
suggest plagioclase. The rock is a metamorphic one, a kind of fine- 
grained gneiss. 
3. (Cunuri, p. 585.) Consists mainly of hornblende grains rather 
irregular in form, with characteristic cleavage and strong dichroism ; 
but associated with these, and apparently partially replaced by them, 
is a mineral which, from absence of dichroism and general aspect, 
more resembles decomposing augite. There are a good many grains 
of quartz and some of decomposing felspar. The rock appears to be 
a hornblende schist with but slight traces of foliation. 
* Quart. Journ. Geol, Soc. vol. xxxiii. p. 896. 
