Clarke finds proofs of the elevation of the land, to the amount of 400feet, at the Cape of Good Hope.("Proceedings of the Geological Society"volume 3 page 420.) In the neighbourhood of the Bay of Islands in New Zealand, I observed that the shores were scattered to some height, as at Van Diemen's Land, with sea-shells, which the colonists attribute to the natives.(I will here give a catalogue of the rocks which I met with near the Bay of Islands, in New Zealand:--1st, Much basaltic lava, and scoriform rocks, forming distinct craters;--2nd, A castellated hill of horizontal strata of flesh-coloured limestone, showing when fractured distinct crystalline facets: the rain has acted on this rock in a remarkable manner, corroding its surface into a miniature model of an Alpine country: Iobserved here layers of chert and clay ironstone; and in the bed of a stream, pebbles of clay-slate;--3rd, The shores of the Bay of Islands are formed of a feldspathic rock, of a bluish-grey colour, often much decomposed, with an angular fracture, and crossed by numerous ferruginous seams, but without any distinct stratification or cleavage.Some varieties are highly crystalline, and would at once be pronounced to be trap; others strikingly resembled clay-slate, slightly altered by heat: I was unable to form any decided opinion on this formation.) Whatever may have been the origin of these shells, I cannot doubt, after having seen a section of the valley of the Thames River (37 degrees S.), drawn by the Rev.W.Williams, that the land has been there elevated: on the opposite sides of this great valley, three step-like terraces, composed of an enormous accumulation of rounded pebbles, exactly correspond with each other: the escarpment of each terrace is about fifty feet in height.No one after having examined the terraces in the valleys on the western shores of South America, which are strewed with sea-shells, and have been formed during intervals of rest in the slow elevation of the land, could doubt that the New Zealand terraces have been similarly formed.I may add, that Dr.Dieffenbach, in his description of the Chatham Islands ("Geographical Journal" volume 11 pages 202, 205.) (S.W.of New Zealand), states that it is manifest "that the sea has left many places bare which were once covered by its waters."KING GEORGE'S SOUND.
This settlement is situated at the south-western angle of the Australian continent: the whole country is granitic, with the constituent minerals sometimes obscurely arranged in straight or curved laminae.In these cases, the rock would be called by Humboldt, gneiss-granite, and it is remarkable that the form of the bare conical hills, appearing to be composed of great folding layers, strikingly resembles, on a small scale, those composed of gneiss-granite at Rio de Janeiro, and those described by Humboldt at Venezuela.These plutonic rocks are, in many places, intersected by trappean-dikes; in one place, I found ten parallel dikes ranging in an E.
and W.line; and not far off another set of eight dikes, composed of a different variety of trap, ranging at right angles to the former ones.Ihave observed in several primary districts, the occurrence of systems of dikes parallel and close to each other.
SUPERFICIAL FERRUGINOUS BEDS.
The lower parts of the country are everywhere covered by a bed, following the inequalities of the surface, of a honeycombed sandstone, abounding with oxides of iron.Beds of nearly similar composition are common, I believe, along the whole western coast of Australia, and on many of the East Indian islands.At the Cape of Good Hope, at the base of the mountains formed of granite and capped with sandstone, the ground is everywhere coated either by a fine-grained, rubbly, ochraceous mass, like that at King George's Sound, or by a coarser sandstone with fragments of quartz, and rendered hard and heavy by an abundance of the hydrate of iron, which presents, when freshly broken, a metallic lustre.Both these varieties have a very irregular texture, including spaces either rounded or angular, full of loose sand: from this cause the surface is always honeycombed.The oxide of iron is most abundant on the edges of the cavities, where alone it affords a metallic fracture.In these formations, as well as in many true sedimentary deposits, it is evident that iron tends to become aggregated, either in the form of a shell, or of a network.The origin of these superficial beds, though sufficiently obscure, seems to be due to alluvial action on detritus abounding with iron.
SUPERFICIAL CALCAREOUS DEPOSIT.
A calcareous deposit on the summit of Bald Head, containing branched bodies, supposed by some authors to have been corals, has been celebrated by the descriptions of many distinguished voyagers.(I visited this hill, in company with Captain Fitzroy, and we came to a similar conclusion regarding these branching bodies.) It folds round and conceals irregular hummocks of granite, at the height of 600 feet above the level of the sea.