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12th. Eagle Hawk Neck. — The arenaceous and siliceous, flaggy, fossiliferous rock, noticed in the vicinity of the river Nive and Mount Wellington, is here found at low-water mark, in almost horizontal strata. It is laid bare by the action of the sea-water on the superincumbent conglomerate, and exhibits a surface fissured in a series of rectangular squares.
This rock is characterized by the abundance of Spirifer vespertilio and Sp. avicula.
MLNERALOGICAL DESCRIPTION OF ROCKS
The porphyries of the two colonies present five varieties which may be named from the varieties of the homogeneous and compact basis in Avhich the crystals or grains of other mineral are embedded.
Var. 1. Felspathic Porphyry.
In the eight localities in which this variety of porphyry occurs, some differences are observable.
On the river Forrester, where the rock pierces through sienite and granite, its colour is greyish; its structure nearly compact; its fracture uneven, and nearly dull: it contains milky crystals of felspar, and highly comminuted grains of mica and hornblende, which latter predominate.
At Waterhouse Point. This porphyry is associated with granite and eurite; its colour is ashy grey, with a faint reddish tinge 'on the weather side, and a darkish cherry on the inside of the fresh broken. It is compact; fracture conchoidal and dull: it contains
grains of limpid quartz, and of flesh red felspar; some of which are rounded, some angular: those Of felspar predominate.
To the South of Waterhouse Point, the felspathic porphyry is associated with eurite. Its colour is flesh red; structure nearly compact; fracture foliated; lustre glittering: it contains grains of hornblende scantily disseminated, and of laminar felspar, which predominate.
The vicinity of Mount Cameron. — This rock is observed between granite and granitic porphyry. Its colour is a yellowish red; structure compact; fracture uneven and dull: the embedded minerals are quartz, mica, and felspar; the last predominating.
In the Black Range, where it is associated with granitic porphyry and eurite, this rock is similar to that found to the S. of Watherhouse Point, excepting that it does not contain any crystals of hornblende.
At St. George's River it is found between granitic porphyry and siliceous slate. Colour yellowish; structure compact; fracture conchoidal and glittering Its embedded crystals are limpid quartz and felspar, the last predominating.
At the river Nive (twenty miles north of Marlborough), this rock is associated with granitic porphyry. Its colour is greyish green; structure compact; fracture uneven and shining. It contains grains of limpid quartz, crystals of hornblende, and rounded crystals of finely laminated felspar.
Before the blow-pipe, the paste of this variety melts into a white enamel, compact and translucent, and sometimes containing bubbles.
Remarks. — The most remarkable fact connected with these seven species of porphyry is, that, different as they seem to be, they are to be found in one current, or rather in one mass of porphyry at Port Stephen, New South Wales.
Var. 2. Petrosilex Porphyry.
Colour brownish black; structure compact; fracture uneven and dull. It contains crystals of felspar and hornblende, with some mica, those of hornblende greatly predominating. It resembles the melaphyre of Brongniart, and before the blow-pipe melts sometimes at the edges only into a blackish, porous enamel: is found at Cape Portland, and on the west coast of Van Diemen'a Land.
Var. 3. Quartzose Porphyry.
In the localities in which this variety of porphyry is found, some difference in its external character is observable.
At Mount Cameron (V.D.L.), where it lies between granite and gneiss, it is of a yellowish colour; structure nearly compact; fracture uneven and at times splintery. It contains grains of limpid quartz, and of felspar minutely comminuted, the grains of quartz predominate.
At Barber's Creek, Modbury, Bango Range, and between DerranguUen and Jugion Creek (N. S. W.); on the dividing range, and at Mount St. Patrick (in V. D. L.), where it is associated with sienite, its colour is grey; structure very compact; fracture conchoidal, splintery and glittering. The contained crystals are limpid quartz, mica and minute grains of hornblende: the quartz predominating.
On the river Forth, the quartzose porphyry is associated with basalt and claystone porphyry: its colour is greenish. It contains crystals or grains of limpid quartz , hornblende, and minutely comminuted mica. Its structure is very compact; fracture splintery, lustre vitreous: the predominant ingredient is quartz.
Before the blow-pipe, the paste of this variety does not melt, except sometimes at the edges only.
Var. 4. Claystone Porphyry.
Occurs at two localities, the specimens of which present marked differences in their external character. That of the river Forth, east of Western Bluff, is associated with trachyte proper and cellular trachyte. Its colour is dark cherry brown; structure very compact; fracture uneven and splintery. It contains grains of glassy quartz, which predominate in the compound, and possess all the characteristics of the quartzose porphyry of Von Buch.
That of the Vale ofBelvoir is associated with basalt and compact limestone. Its colour, like the preceding sub-variety is a blackish cherry brown; its structure compact: the paste, however, when examined with a glass, is found to consist of small shining grains in a state of vitrification. The fracture, though splintery, is more even than that of the river Forth: it contains felspar of a dull milky hue, which is sometimes rounded, sometimes angular. Subjected to the bloAv-pipe it melts at the edges only: that of the last-mentioned locality resists even the heat produced by oxygen gas.
This claystone porphyry occupies a larger extent of country than any of the other varieties; indeed, the whole range which separates the Yale of Belvoir from Mayday Plains is composed of the Belvoir porphyry, while that of the river Forth stretches on the eastern side of Western Bluff, towards the Eldon range.
Var. 5. Mimophyre.
This variety of porphyry is composed of grains of felspar, quartz, and at times of mica, embedded in an argillaceous cement. It occurs in New South Wales, south of Lake Burraburra, and on the banks of Mitta-mitta Kiver. It passes into psephite, an argillaceous and sandy paste, which cements grains or fragments of mica slate, argillite, and quartz, irregularly interspersed.
Remarks. — Every where in the vicinity of the five above varieties of porphyries, phenomena of disturbance and disorder are perceptible, impressing the mind with an idea of that high degree of force and violence, by which they had been injected between the stratified and unstratified rocks. The porphyry of the river Nive, twenty miles north of Marlborough, seems to have exerted a greater chemical power than any of the others, having transformed a sedimentary rock of non-fossiliferous greywacke into a mass of porphyritic structure, and burst through the fossiliferous greywacke, and covered it with breccia, composed of fragments of quartz rock and mica slate. The porphyry of the river Forth is scarcely less remarkable for the extensive changes which its intrusion effected, or rather, which immediately after that intrusion were worked out. These two kinds of porphyry, as well as the others here noticed, when propelled from beneath, so convulsed, tore, and shattered the superincumbent rocks, that the crust thus loosened and weakened, became as it were a beaten track prepared for the subsequent intrusion of greenstone, basalt, and trachyte. Indeed the porphyritic ejections have given such facilities for the intrusion of other igneous rocks, that it is almost always from the vicinity of their eruption that greenstone, basalt, and trachyte appear to have spread, and now cover immense tracts of the country.
Diabase (Brongniart). Diorite (Haiiy).
The varieties of this kind of rock, belonging to the second epoch, are uniformly composed of felspar and