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hornblende in the state of grains or of small crystals, in proportions somewhat different, but in which the hornblende constantly predominates. They vary also in their structure, which is either slaty, prismatic, or amorphous; as, however, that structure has been the result of peculiar agencies, acting evidently at very different geological epochs, their -description and examination is given here separately.
Var. 1. Slaty Greenstone, Diabase schistoide.
Its invariable colour in the recent fracture, is between a leek and pistachio green; that of the exterior of the rock is reddish brown. The internal surface has a waxy lustre; the imbedded crystals of hornblende are generally brilliant.
Its structure is schistose, but the layers are never parallel; and are running from a thickness of two or three inches to a wedge-like termination. For the most part, these seams present a lenticular form resembling convex lenses closely fitting, and thus beautifully illustrating the successive overflowings of the incandescent matter. It does not adhere to the tongue, and exhales an argillaceous odour. The streak varies; the powder obtained by trituration is of a brownish yellow colour. The structure is compact and hard; the blow of the hammer on the mass merely detaches layers which exhibit surfaces of their own. The shape of the fragments is commonly tabular.
Localities. — It is found at Booral on the Upper Hunter in Argyleshire, and about Lake George. Its greatest extent occurs in the locality of the Upper Hunter, on the Liverpool Range, of which it forms the culminant point.
In Van Diemen's Land, it is found in every part of the island. The localities which supply the most important facts bearing upon its geological relation are between Launceston and Mount Direction; Mount Direction and George Town; George Town and Stony Head; Cape Portland, St. Patrick's Head; between the Break-o'-Day River and the Tyne; Ben Lomond, Ben Nevis, Port Sorrel, Dry's Bluff, Lake Arthur, Lake Sorrel, the Great Lake, Lake St. Clair, Western Bluff, Mount Cradle; the source of the Nive and Mount Cameron West.
Remarks. — This variety of greenstone occurs at various heights above the sea and the shore, capping some of the most prominent elevations of the interior of the island. The greatest height which this greenstone attains is 5200 feet. It is invariably and intimately associated with porphyries, argillaceous schist, mica slate, sienite, granite, silicious slate, and limestone; when it is isolated from the prismatic or amorphous greenstone, its seams are horizontal. When, however, these varieties are in contact with it, the seams are vertical, broken, and distorted.
The examination of the great area which this schistose greenstone covers in Van Diemen's Land, leads to the discovery of sources from which it overflowed the island. The principal sites appear to have existed in the vicinity of Cape Portland; between Mount Barrow and Mount Arthur; on the north side of Ben Lomond, on Mount St. Patrick, at Port Sorrel, on Mount Cradle, Mount Cameron West; and at the source of the Nive.
In all these places, the schistose greenstone is associated with porphyries. This association of the two rocks strongly inclines me to believe that the slaty greenstone was erupted or propelled along the preexisting side or slope of the consolidated porphyry,
Var. 2. Prismatic Greenstone.
Its colour in the recent fracture is blackish green; on the surface, yellowish brown. The lustre of the paste is waxy; that of the hornblende which it contains vitreous; it does not adhere to the tongue, and exhales an argillaceous odour; its streak is dissimilar and dull; its colour a brownish grey; when struck with the hammer, it gives a metallic sound: it is compact, hard, its fracture is somewhat conchoidal. The structure is prismatic, the prisms having three, four, five, six, or seven sides. Their diameter varies from three to eight feet; the length of two or three columns, which are still entire, exceeds 100 feet. The clustered columns are sometimes very closely united; sometimes they are only in close contact, and are separated by the fall of the masses. Some of the columns have but a slight influence upon the magnetic needle; and in these the axes range east and west. The columns lying parallel with the meridian, or nearly so, disclose a strong polarity; a phenomenon worth noting, as the property seems to be more dependent on the bearings of the axes of these columns than on their constituents. The discovery of this polarity was consequent upon the anomalous results which the observations of the magnetic intensity furnished me by the prismatic greenstone on Ben Lomond.
Var. 3. Amorphous Greenstone.
Its colour is like that of the preceding varieties, the fracture displaying sometimes a blackish green, sometimes a leek green; the exterior is invariably a yellowish brown.
Its paste has a waxy lustre. Its structure is amorphous; the fracture is somewhat splintery and
uneven; the shape of the fragments irregular. It has a dull sound when struck with the hammer.
Localities. — In New South Wales, this rock is found west of Port Stephen; also in the Liverpool and Honey-suckle Range, at Modbury and in Argyleshire. In Van Diemen's Land, it is distributed widely over the island; the principal localites are between Launceston, Mount Direction, and George Town, at Cape Portland, on Ben Lomond, at the junction of the North Esk with Ben Lomond Creek, between Quamby's and Port Sorrel, at Dry's Bluff, Miller's Bluff, Scrubby Den, Lake Arthur; between Lake Arthur and the Great Lake, at Lake St. Clair, on Eldon Range, Mount Cradle, Barn Bluff, Mount Roland, Mount Wellington, Tasman's Peninsula, Research Bay, Adamson's Peak, Bruin, Green Island (Bass's Straits), and Woolnorth, Cape Grimm, and Mount Cameron, West.
Basalt, Lava, And Trachyte.
From my own observations made among the volcanos of Europe, Mexico, and South America, and more particularly in the tremendous volcanic laboratory of Kirauea in the Sandwich Islands, I am inclined to believe that there will be found insuperable difficulties in the way of a classification of volcanic products.
In many instances, the existing subdivision of volcanic rocks into varieties is but imaginary — the distinctions referring rather to individual specimens than to the mass from which they were taken. Thus the three varieties into which basalt has been subdivided, and each of which is characterised by the preponderance either of labradorite, of orthoze, or of albite, have been found by the writer in one current of basalt (at Kirauea), within an area of four cubic feet; some splinters of these being soluble in hydrochloric acid, while others were not.
Again, the four varieties of lava, distinguished by Dolomieu and others into granitolte, porphyritic, micaceous, and hornblende trachyte, with still other varieties, is inadmissible from the fact that these varieties at times constitute a part of the continuous mass of rock, with such gradual transition, that it is impossible to assign the boundary where the one variety begins and the other terminates.
In illustration of this subject, I shall give here a brief description of. the volcano of Kirauea, extracted from my manuscript notes.
"The volcano of Kirauea lies on the north-western side of Mouna Loa, about twenty miles from the summit of that mountain, and about forty from the Bay of Hilo: its latitude, determined on the spot, is 19° 27'. Its present size surpasses that of every other known volcano, yet it now hardly displays more than a third of its pristine grandeur. Like some of the old Egyptian cities, Kirauea has no other chronicles of the past than a part of its ancient walls still standing, and a part either in ruins, or buried at some period beyond the memory of man, under the ashes of successive eruptions, though still to be recognised and traced by means of the masses which stand at intervals as land-marks.
"When, pencil in hand, we take the circuit of these land-marks,—collect, as it were, the scattered materials, fill up the breaches, and thus reconstruct the former orifice of the crater,—we are thrilled with awe at the contemplation.
"Fearful and astonishing must have been the action of this volcano in the days of its former greatness,— when it belched its fires from a mouth twenty-four miles in circumference, and overwhelmed the country with its devouring floods. But as all power bears