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The large Brumer Island is composed of igneous rocks as formerly mentioned; and at Dufaure Island I obtained from some canoes which came off to us a few smooth water-worn pieces of hornblendic porphyry.
In this case and at Rat Island, the passage of the gneiss into imperfect hornblendic or into chloritic slate, seemed to be connected with the segregation of the veins of quartz. Greenough page 78 "Critical Examination" etc., observes that quartz in mica-slate sometimes appears in beds and sometimes in veins.
The hornblendic cement, with its iron or manganese base, produces the variegated appearance which may be seen in specimens from different localities. As may be imagined from their composition, these rocks are as a rule extremely unalterable by ordinary atmospheric agencies, and are susceptible of a high degree of polish, which they retain with the utmost tenacity.
Further north, however, to the west of this great range, there is no clay-slate, but only gneiss, mica, and hornblendic slates, resting on great barren hills of true granite, and forming a tract about sixty miles in width. Fitton in "Geological Proceedings" volume 1 page 29; also some observations by Captain Fitzroy "Voyages" volume 1 page 375. I am indebted also to Mr.
Near Monte Video, where the stratification, as it would be called, of the metamorphic series is, in most parts, particularly well developed, being as usual, parallel to the foliation, we have seen that a mass of chloritic schist, netted with quartz-veins, is entangled in gneiss, in such a manner as to show that it had certainly originated in some process of segregation: again, in another spot, the gneiss tended to pass into hornblendic schist by alternating with layers of quartz; but these layers of quartz almost certainly had never been separately deposited, for they were absolutely continuous with the numerous intersecting veins of quartz.
Hornblende a black or dark green mineral, an iron-magnesian silicate, about as hard as feldspar is sometimes found as a fourth constituent, and the rock is then known as HORNBLENDIC GRANITE. Granite is an acidic rock corresponding to rhyolite in chemical composition.
Near M. Video, the space which I more accurately examined was about fifteen miles in an east and west line, and here I found the foliation of the gneiss and the cleavage of the slates generally well developed, and extending parallel to the alternating strata composed of the gneiss, hornblendic and chloritic schists.
Before we came to the North Platte, we were astonished by a ship, equalling the Great Eastern in size, even surpassing it in beauty of outline, its masts of columnar sandstone snapped by a storm, its prodigious hulk laboring in a gloomy sea of hornblendic granite, its deck-houses, shapen with perfect accuracy of imitation, still remaining in their place, and a weird-looking demon at the wheel steering it on to some invisible destruction.
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.
The valley of the Nile from Cairo to Assouan has yielded a series of objects in flint, porphyry, and hornblendic rock, retaining traces of human workmanship, and reminding us of similar implements of European type.
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