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Updated: May 6, 2025
ACTINOLITE SCHIST is a slaty foliated rock, composed chiefly of actinolite, an emerald-green mineral, allied to hornblende, with some admixture of garnet, mica, and quartz. MICA-SCHIST or MICACEOUS SCHIST is, next to gneiss, one of the most abundant rocks of the metamorphic series. It is slaty, essentially composed of mica and quartz, the mica sometimes appearing to constitute the whole mass.
DIORITE, still less siliceous, is composed of hornblende and feldspar, the latter mineral being of different variety from the feldspar of granite and syenite. GABBRO, a typical basic rock, corresponds to basalt in chemical composition.
This cement is composed of angular quartz-fragments, broken from the reefs or veins, and fragments of diorite and hornblende schists, cemented together by lime; it is very hard and solid and, in places, continues to a depth of over twenty feet. The gold is extracted from these depths by crushing and dry-blowing.
As at Chilolé hill, we have igneous rocks, apparently trap, capped with masses of beautiful white dolomite. We still ascend in altitude as we go westwards, and come upon long tracts of gneiss with hornblende. The gneiss is often striated, all the striae looking one way sometimes north and south, and at other times east and west.
The geologists were interested, and we others learnt something of olivines, green in crystal form or oxidized to bright red, granites or granulites or quartzites, hornblende and feldspars, ferrous and ferric oxides of lava acid, basic, plutonic, igneous, eruptive schists, basalts &c. All such things I must get clearer in my mind. Tuesday afternoon a cold S.E. wind commenced and blew all night.
The green slates include some strata of grunstein, and even contain balls of that substance. I nowhere saw the green slates alternate with the black slates of the ravine of Piedras Azules: at the line of junction these two slates appear rather to pass one into the other, the green slates becoming of a pearl-grey in proportion as they lose their hornblende.
It was also found by analysis that augite usually contained more lime, less alumina, and no fluoric acid; which last, though not always found in hornblende, often enters into its composition in minute quantity. In addition to these characters, it was remarked as a geological fact, that augite and hornblende are very rarely associated together in the same rock.
I observed Pegmatite of a white colour, and hornblende Porphyry and Diorite. A shrubby Clerodendron and an arborescent Bursaria, covered with white blossoms, adorned the forest. The latitude was 20 degrees 0 minutes 36 seconds. April 11. We continued our journey up the river, in a W.N.W. direction, for about ten miles. The first part of our journey lay through a most beautiful country.
I did not ascend Mount McConnel, but it seemed to be composed of a species of domite. On the subordinate hills I observed sienite. The bed of the river furnished quite a collection of primitive rocks: there were pebbles of quartz, white, red, and grey; of granite; of sienite; of felspathic porphyry, hornblende, and quartz-porphyry; and of slate-rock. The morning was cloudless.
Lavas are chiefly composed of three varieties of feldspar, varying in specific gravity from 2.4 to 2.74; of hornblende and augite, varying from 3.0 to 3.4; of olivine, varying from 3.3 to 3.4; and lastly, of oxides of iron, with specific gravities from 4.8 to 5.2.
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