Near Hobart Town, I observed one dike, nearly a hundred yards in width, on one side of which the strata were tilted at an angle of 60 degrees, and on the other they were in some parts vertical, and had been altered by the effects of the heat.On the west side of Storm Bay, I found these strata capped by streams of basaltic lava with olivine; and close by there was a mass of brecciated scoriae, containing pebbles of lava, which probably marks the place of an ancient submarine crater.Two of these streams of basalt were separated from each other by a layer of argillaceous wacke, which could be traced passing into partially altered scoriae.The wacke contained numerous rounded grains of a soft, grass-green mineral, with a waxy lustre, and translucent on its edges: under the blowpipe it instantly blackened, and the points fused into a strongly magnetic, black enamel.In these characters, it resembles those masses of decomposed olivine, described at St.Jago in the Cape de Verde group; and I should have thought that it had thus originated, had I not found a similar substance, in cylindrical threads, within the cells of the vesicular basalt,--a state under which olivine never appears; this substance, I believe, would be classed as bole by mineralogists.(Chlorophaeite, described by Dr.
MacCulloch ("Western Islands" volume 1 page 504) as occurring in a basaltic amygdaloid, differs from this substance, in remaining unchanged before the blowpipe, and in blackening from exposure to the air.May we suppose that olivine, in undergoing the remarkable change described at St.Jago, passes through several states?)TRAVERTIN WITH EXTINCT PLANTS.
Behind Hobart Town there is a small quarry of a hard travertin, the lower strata of which abound with distinct impressions of leaves.Mr.Robert Brown has had the kindness to look at my specimens, and he informed me that there are four or five kinds, none of which he recognises as belonging to existing species.The most remarkable leaf is palmate, like that of a fan-palm, and no plant having leaves of this structure has hitherto been discovered in Van Diemen's Land.The other leaves do not resemble the most usual form of the Eucalyptus (of which tribe the existing forests are chiefly composed), nor do they resemble that class of exceptions to the common form of the leaves of the Eucalyptus, which occur in this island.
The travertin containing this remnant of a lost vegetation, is of a pale yellow colour, hard, and in parts even crystalline; but not compact, and is everywhere penetrated by minute, tortuous, cylindrical pores.It contains a very few pebbles of quartz, and occasionally layers of chalcedonic nodules, like those of chert in our Greensand.From the pureness of this calcareous rock, it has been searched for in other places, but has never been found.
From this circumstance, and from the character of the deposit, it was probably formed by a calcareous spring entering a small pool or narrow creek.The strata have subsequently been tilted and fissured; and the surface has been covered by a singular mass, with which, also, a large fissure has been filled up, formed of balls of trap embedded in a mixture of wacke and a white, earthy, alumino-calcareous substance.Hence it would appear, as if a volcanic eruption had taken place on the borders of the pool, in which the calcareous matter was depositing, and had broken it up and drained it.
ELEVATION OF THE LAND.
Both the eastern and western shores of the bay, in the neighbourhood of Hobart Town, are in most parts covered to the height of thirty feet above the level of high-water mark, with broken shells, mingled with pebbles.The colonists attribute these shells to the aborigines having carried them up for food: undoubtedly, there are many large mounds, as was pointed out to me by Mr.Frankland, which have been thus formed; but I think from the numbers of the shells, from their frequent small size, from the manner in which they are thinly scattered, and from some appearances in the form of the land, that we must attribute the presence of the greater number to a small elevation of the land.On the shore of Ralph Bay (opening into Storm Bay) I observed a continuous beach about fifteen feet above high-water mark, clothed with vegetation, and by digging into it, pebbles encrusted with Serpulae were found: along the banks, also, of the river Derwent, Ifound a bed of broken sea-shells above the surface of the river, and at a point where the water is now much too fresh for sea-shells to live; but in both these cases, it is just possible, that before certain spits of sand and banks of mud in Storm Bay were accumulated, the tides might have risen to the height where we now find the shells.( It would appear that some changes are now in progress in Ralph Bay, for I was assured by an intelligent farmer, that oysters were formerly abundant in it, but that about the year 1834 they had, without any apparent cause, disappeared.In the "Transactions of the Maryland Academy" volume 1 part 1 page 28 there is an account by Mr.Ducatel of vast beds of oysters and clams having been destroyed by the gradual filling up of the shallow lagoons and channels, on the shores of the southern United States.At Chiloe, in South America, Iheard of a similar loss, sustained by the inhabitants, in the disappearance from one part of the coast of an edible species of Ascidia.)Evidence more or less distinct of a change of level between the land and water, has been detected on almost all the land on this side of the globe.
Captain Grey, and other travellers, have found in Southern Australia upraised shells, belonging either to the recent, or to a late tertiary period.The French naturalists in Baudin's expedition, found shells similarly circumstanced on the S.W.coast of Australia.The Rev.W.B.
