« AnteriorContinuar »
molten; while a part still remained lodged against the sides of the cauldron, and is now seen as a rock 200 feet in height, exhibiting basalt, trachyte, and lava of several varieties. Between the scoriaceous lava, approaching to slag, which is uppermost, and the close-grained basalt, which forms the lowest portion of the rock, the transition is so gradual, that it is impossible to assign the spot where basalt ceases and trachyte or lava begins."
The enlightened Von Buch has remarked*, and Dufresnoy and Elie de Beaumont have confirmed the observation, that the word lava is an expression which relates only to the form. The facts collected in the crater of Kirauea would lead one to suppose that the words basalt, trachyte and lava, serve only to distinguish the upper from the lower part of a molten matter. It is probable also that the distinction of basalt into columnar and amorphous refers only to their relative form, and that both rocks belong to the same basaltic current, and most likely resulted from the angle of inclination of the plane or surface which that current has overflowed.
Thus, on the road from the heads of Cowrang Creek (New South Wales) to Lake Omeo, there is a basaltic, horizontal dyke, running from south to north. At the left bank of the river Mitta-Mitta, which bank is about 100 feet high, that dyke is seen precipitating itself downwards into the river, and thus appears like a frozen or petrified cascade. The dyke throughout its horizontal course presents an amorphous form; in its downward fall it assumes insensibly the form of columnar concretions, till it reaches the bottom of MittaMitta, where it exhibits three, four, six, and seven-sided regular prisms, not exceeding three inches in diameter.
The localities at which basalt and its varieties occur
* In describing the tract between Lake Orta and Lake Lugano.
in the two colonies, are the following:—Port Stephens, the Lower and Upper Hunter, Mount Tomah, Mount Hay, Mount King George, between Bathurst and Boree; Frederic Valley, Mount Canoblas, between Molong and Wellington Valley; on the Razor Back range, at Illawara; on the crest of the Mitagong range, at Lake George, Shoalhaven, Dutzton at Lake Omeo, and the river Mitta-Mitta, (New South Whles). Kent's Group, Green and Swan islands, (Bass Straits.) Between George Town and Stony Head; at the Gardens; Ben Lomond; Vale of Belvoir; between Gadd's Hill and Middlesex Plains; Hampshire Hills; the Duck river; the Welcome river; Cape Grimm; Mount Cameron West; at Arthur's Lake; and the source of the river Nive; at Lake St. Clair; on Mount Cradle; between Brighton and Bridgewater; Mount Wellington; HobartTown; Research Bay; EsperanceHarbour; Bruni'sIsland, andTasman'sPeninsula; ( VanDiemen's Land).
These rocks present two varieties: the first consisting of aggregated fragments, which preserve the character of the rocks from which they are derived; while in the second this character is entirely effaced, both the paste and the fragments passing through different stages of change, and assume at last the appearance of uniform pitchstone or jasperoid rocks.
First Variety An aggregate of unaltered mica
slate, argillite, quartz rock, and felspar. The predominant colour is usually a shade of ash-grey. The cement seems to be composed of the same materials as the fragments, but consists of grains so minute that it resembles a homogeneous paste.
The Second Variety of breccia presents the appearance of a semi fused compound of variegated colour. Its lustre is resinous; it is translucent at the edges; compact, and extremely hard; having sometimes a splintery, sometimes a conchoidal fracture. This variety is frequently amygdaloidal, and though different in external character from the first variety, leaves no doubt that both were originally identical. They generally occur together, amongst rocks of the first or second epoch, in large masses. In New South "Wales, the second variety is observed only at St. Patrick's Plains, Wellington Valley, and Lake George. In Van Diemen's Land, the two varieties crest the ridge which connects Ben Lomond and Ben Nevis, an elevation of 3200 feet; the first variety, incumbent on mica slate, quartz rock, argillite, and granite, is superposed by the second, which is connected with hornblende rocks. It occurs also at Waterhouse Point, between the river Tamar and Mersey, at Table Cape, Hampshire Hills; in the Vale of Belvoir; Dry's Bluff, Lake Mills river; in all the above localities the breccias are associated with eurite, quartz rock, and clay slates.
Var. 1. Foliated granular Limestone. (Jameson.) Chaux carbonate saccaroide. (Brongniart.)
That variety presents itself in different shades, from black to snow-white: these shades are sometimes uniform in the mass; sometimes they occur in spots, veins, or clouds of different hues. Its structure is both foliated and granular: in some cases the grains are only discernible by means of a lens. It is generally a pure carbonate of lime, and very seldom with admixture of foreign ingredients. In two localities only, it contains crystals of hornblende and shorl.
Localities—In New South Wales granular limestone is extensively developed. It is found on the Upper and Lower Hunter, between Wellington and Mount Canoblas; between Cullen-bullen and Wolerowang; on the Wollondilly; in Westmoreland; on the Shoalhaven river, between Amprier and Barber's Creek; at Lake George, Yass Plains, and Murrumbidgee; on the Murray and on the river Thompson (Gipps Land). In Van Diemen's Land, it is found south-east of Mount Horror; on Asbestos Hills; at Circular Pond marshes, Belvoir Vale, House Top Tier, and the sources of the river Nive.
Some parts of New South Wales can boast of most beautiful marbles, very valuable for statuary and other ornamental purposes; as on the Wollondilly, where the rock is as closely grained and as white as the Carrara marble; and at Amprier, Shoalhaven, where the stone is a jet black traversed by veins of a white calcareous spar: between Wellington Valley and Boree there are also innumerable varieties of finely variegated marbles, in which caves are found of the greatest interest to geology. The most remarkable in New South Wales are those in the neighbourhood of Wellington, Boree, Shoalhaven, and Murrumbidgee. In Van Diemen's Land such caves are limited to the river Mersey, and to Circular Pond marshes. The latter locality deserves to be noticed, as the funnel-like shape of its surface, combined with the form of the caves, would lead to the bebef that the limestone tract has undergone partial subsidencies.
Var. 2. Compact Limestone. (Kirwan.)
Common compact Limestone. (Jameson.)
Is in colour as variable as the preceding variety: in the greater number of localities the ash-grey tint predominates. It is at times massive, at times has a somewhat stratified appearance. It is not a pure carbonate, but contains many foreign ingredients, and passes into the earthy variety of limestone. It resembles very closely the preceding variety, and in most cases, like the limestone of Boree, is intimately associated with it. It is this variety that contains the marine remains which have been already enumerated.
Localities.—In New South Wales this limestone is found at St. Patrick's Plains, Illawara, Shoalhaven, Yass Plains, Boree, and the river Thompson (Gipps Land).
In Van Diemen's Land the limestone belonging to this formation occupies both sides of the dividing range; and although much divided by the igneous rocks, it may, in all the localities, be still identified by the organic remains. The track of the western belt is indicated in four conspicuous localities, First, the Break-o'-day Valley, where the limestone lies at an altitude of 700 feet, incumbent on greywacke, and associated with greenstone. It is mostly massive, and contains numerous organic remains, and fragments of older rocks. The second locality, between Mona Vale and Ross, is at the height of 600 feet above the sea, where the limestone is associated with trachyte, and is fossiliferous; the third is northeast of Campbeltown; the fourth is on the west arm of the Tamar, where it is associated with greywacke and granular limestone. The eastern belt has also several localities where this limestone rock appears. First, the sources of the river Nive, where it is connected with granular limestone, and an arenaceous fossiliferous rock; second, the Eastern Marshes, where it appears in great fossiliferous masses, much fractured and dislocated by the intervening greenstones; third, the foot of Table Mount, where it is equally fossiliferous ; fourth, Mount Dromedary ; fifth, Mount Wellington. In the three last-mentioned localities the limestone contains many most interesting