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Updated: October 6, 2025
When a small fragment is heated in the blowpipe, the black specks are easily fused into black brilliant beads, which become magnetic, characters that apply to no common mineral except hornblende or augite. With the black specks there are mingled some others of a red colour, which are magnetic before being heated, and no doubt are oxide of iron.
The mica in recent volcanic rocks, gabbros, and diorites is usually Biotite, while that so common in metamorphic limestones is usually, if not always, Phlogopite. Amphibole is a general name for all the different varieties of Hornblende, Actinolite, Tremolite, etc., while Pyroxene includes Augite, Diallage, Malacolite, Sahlite, etc.
It is a condition eminently of combination, for such rock is invariably composed of two or more of four substances silica, mica, quartz, and hornblende which associate in it in the form of grains or crystals, and which are themselves each composed of a group of the simple or elementary substances.
In the hand specimen it is an apparently pure orthoclase but in the thin section small scattered quartz grains are observed; as well as the alteration products, Kaolin and sericite. The minerals at contact are quartz, biotite, magnetite and hornblende. Both the quartz and orthoclase contain dust inclusions and crystallites, while the evidences of shearing and crushing are abundant.
The observation he now obtained only confirmed the skipper's previous impression that we were on Herschel Island, one of the Hermite, or Cape Horn group, the mountainous peaks of which are mainly composed of green stone, in which hornblende and feldspar are more or less conspicuous, and the presence of iron very apparent, some of the rocks being intensely magnetic, causing the needle of a little pocket compass I had to execute all sorts of strange freaks.
Near the place of our first encampment on the Lynd, in lat. 17 degrees 58 minutes, I observed a sienite, to which the distribution of the hornblende in layers had given the stratified appearance of gneiss. Another rock was composed of felspar and large leaflets of white mica, or of quartz and white mica.
It was also remarked that in the crystalline slags of furnaces augitic forms were frequent, the hornblendic entirely absent; hence it was conjectured that hornblende might be the result of slow, and augite of rapid cooling. This view was confirmed by the fact that Mitscherlich and Berthier were able to make augite artificially, but could never succeed in forming hornblende.
Unquestionably the facts and experiments above mentioned show the very near affinity of hornblende and augite; but even the convertibility of one into the other, by melting and recrystallising, does not perhaps demonstrate their absolute identity. For there is often some portion of the materials in a crystal which are not in perfect chemical combination with the rest.
Lastly, Gustavus Rose fused a mass of hornblende in a porcelain furnace, and found that it did not, on cooling, assume its previous shape, but invariably took that of augite.
It is probable that, had they been subjected to more intense Plutonic action, they would have been transformed into hornblende- schist, foliated chlorite-schist, scaly talcose-schist, mica-schist, or other more perfectly crystalline rocks, such as are usually associated with gneiss.
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