6. NOTE upon a RECENT DISCOVERY of TIN-ORE in TASMANIA. By CHARLES GOULD, Esq., B.A., F.G.S. (Read June 24, 1874.)

As a sequel to discoveries of tin-ore in abundance made during the last two years in Queensland and New South Wales, there has been one of some importance recently in Tasmania, which has this special interest, that the ore has been found in large masses in situ, and that the containing rock is lithologically distinct from the ordinary ternary granite which forms the whole of the stanniferous country of Queensland, and, so far as my information extends, of New South Wales.

The tin-stone also has an individuality of character not shared by that in the adjacent colonies, occurring in several distinct varieties of form within the distance of a few chains.

I forward a small series of samples for the inspection of the Members of the Society, among which I would specially invite attention to a fragment from a large mass (itself 80 lbs. in weight) which has an appearance on fracture sufficiently approaching to that of a hæmatite to deceive a casual observer.

This discovery of tin-ore has been made at Mount Bischoff, in the N.W. portion of Tasmania. Active operations were only commencing when I visited the spot; and in the absence of mining experience on the part of those engaged, coupled with the numerous obstacles to the progress of pioneers, presented by a rough and exceptionally difficult country, some time may be expected to elapse before its full importance is developed.

Mount Bischoff is distant by the present route fifty-four miles from the place of shipment; and while its vicinity generally is covered with thick forests of myrtle (Fagus Cunninghami), the summit itself (where the tin occurs) is so densely shrouded by an almost impenetrable scrub of Anodopetalum biglandulosum ("horizontal scrub" of the colonists) that any estimate of the exposed area of stanniferous rock is impossible at present. It does not appear to exceed a few hundred acres.

Mount Bischoff is a conical eminence, rising to about 2500 feet above the level of the sea, perched on the western edge of the great basaltic plateau of the Surrey Hills, the average elevation of the latter being probably a little over 2000 feet.

It consists of a small protrusion of a porphyritic rock having a felsitic base, with granules and crystals of quartz and felspar; it weathers white, and is honeycombed or vesicular on the surface, most probably from the decomposition and removal of pyrites, which is freely disseminated throughout in places.

The western and southern flanks expose uplifted contorted schists, and metamorphosed formations, among which I noted traces of conglomerates belonging to the lowest of the sedimentary deposits in Western Tasmania.

The basalt on the east in places extends right up to, and rests directly on, the porphyry.

Large quantities of what, in the absence of a better term, may be called stream-tin have been already obtained; and the amount will be much increased when an effective water-supply has been provided and the primitive mode of washing, at present employed, abandoned.

This alluvial tin has not been subjected to the action of the running of the water of brooks, but is procured from the shallow surface-drifts resting on the flanks of the mountain, and is derived directly from the disintegration of veins and strings of ore disseminated through the porphyritic rock.

This latter contains oxide of tin, in small veins in irregular bunches, upon joint faces, and, as I also anticipate, will prove to be the case in large deposits in true lodes.

Gossany outcrops of lodes or irregular deposits occur of great extent in places; and in these are met with ferruginous agglomerations of minute particles of tin-stone, forming projecting boulder surface-masses of great size. Nuggets (as they are locally termed) or shoad-stones of rich tin-ore are freely discovered, varying up to four or five hundredweight in size.

Surface-indications of large lodes are also seen, in which a clayey material is traversed by strings of galena, of iron pyrites, and by large quantities of carbonate of iron; no attempt has yet been made to ascertain their nature or develop them.

Unquestionable lodes in the adjacent slaty rocks contain antimony and zinc blende. The average direction of those yet discovered is from 10° to 20° to west of north, and east of south.

I regret that the brevity of my stay at Mount Bischoff, and the undetermined character of the main deposits of ore, prevent me from at present giving fuller details of what will, I think, prove to a very important discovery.

7. An ACCOUNT of a WELL-SECTION in the CHALK at the north-end of DRIFFIELD, EAST YORKSHIRE (supplementing the writer's previous paper*). By R. MORTIMER, Esq. (Read March 25, 1874.)

(Communicated by W. Whitaker, Esq., B.A., F.G.S.)

[Abridged.]

THIS well was dug in the spring of 1873. The site is on ground at at an elevation of 120 feet above the level of the sea, and slopes gently towards the south. The first 7 feet of the section was through the feather-edge of a bed of clay of late glacial age, in which, at a depth of 6 feet, lay a large and irregularly formed boulder of trap-rock, with angles but slightly worn; there were portions of several other rocks. Under this clay is chalk in a very rubbly and broken-up state, to a depth of from 3 to 4 feet, after which it gradually loses the broken-up appearance, and assumes the horizontal laminated form. The chinks and fissures in the upper portion of the chalk were stained to a depth of 8 feet with argillaceous matter from the percolation of rain-water through the overlying clay; but beneath this depth there was no colouringmatter visible from above.

At

The portion of the section which passed through chalk was 47 feet in depth. A careful examination showed that horizontal lamina existed from top to bottom, and varied in thickness from of an inch to 16 inches. From a depth of 20 feet to the bottom, the sides of the well exposed many nearly vertical partings, or faces, in the chalk, where horizontal striae were beautifully shown, covering several square feet together. One large buttress-formed piece exhibited two sides of a square elaborately filled with horizontal striæ. a point about two thirds of the depth of the well, the face of the chalk showed the same kind of markings, but in this case making an angle of about 45 degrees with the horizon. These striated fissure-like partings in the chalk, many of which appear to have never been firmly united with the mass, ran in all directions. In one place several feet of striæ ran from north to south, while above and below these facets pointed in every direction. No appearance of flint was observed.

Intimately connected with, and spread between the hard chalk laminæ were numerous thin horizontal layers of a softer and muddycoloured substance (fuller's earth), of the consistency of compact clay. This material, varying in thickness from of an inch down to a mere film (though in some of the chalk-pits in the neighbourhood it is occasionally from 2 to 3 inches thick), is here found between all the horizontal layers of the chalk. At the depth of 31 to 38 feet there are three beds about 3 feet apart, and measuring from an inch to of an inch in thickness. This fuller's earth, when first taken from the rock, is of a tenacious nature, but in * Quart. Journ. Geol. Soc. vol. xxix. (1873) p. 417.

drying becomes more friable, and frequently breaks up into scaly pieces, which warp and curve. It seems to be composed of comminuted chalk and shells mixed with decayed animal and vegetable remains; in short it consists, for the most part, of the wreck of oceanic life; and a comparison of the two chemical analyses given below, which were made by a well-known analytical chemist (Mr. Thomas Hodgson, of Under Cliff, Bradford), strongly supports this belief:

The author adduces the facts above alluded to in support of his hypothesis that the solid chalk is built up by organisms of a zoophytic nature, and regards the clayey layers as caused by temporary disturbances of the water by which the ordinary quiet" secretion" of the chalk was interrupted.

DISCUSSION.

Prof. HUGHES thought that the origin of the particular structure referred to by Mr. Mortimer was a question for mineralogists and chemists, though somewhat similar forms might be produced mechanically. He pointed out that its occasional occurrence along fissures not in the line of bedding was quite conclusive against its being in any way referable to the period of original deposition. As to the more clayey bands, he considered the Chalk, though often composed in a great measure of comminuted organisms, to be all sedimentary, and had therefore no difficulty in explaining how more clayey matter might accumulate over various areas at different times.

Mr. WILTSHIRE mentioned that argillaceous bands extending over wide areas are to be observed in the Chalk.

Mr. WHITAKER stated that clay bands occur near Beachy Head. He considered that the argillaceous bands are not seen in hard chalk. The analyses given by the author were of much value.

Mr. CHARLESWORTH remarked that the Yorkshire Chalk presents exceptional characters. The sponges found fossilized in it are silicified throughout, but the silica is confined to the sponges, and does not invest them.

Mr. KоCH stated that near Nice the Dolomitic Limestone sometimes exhibits a structure similar to that described in the paper, and that when this structure occurs minute crystals can be detected in the mass.

8. On SLICKENSIDES or ROCK-STRIATIONS, particularly those of the CHALK. By Dr. OGIER WARD. (Read March 25, 1874.)

(Communicated by Prof. Morris, F.G.S.)

THIS subject has been so recently brought under the notice of the Society that I need not describe the appearances; but as there seems to be a most extraordinary divergence of opinion as to their cause I have used the general term of "slickensides," meaning "slipping of surfaces."

In the cretaceous strata of the neighbourhood of Eastbourne these striations may almost be measured by acres, almost every block in the Holywell quarries having its sides either striated or polished; and in the angles of the blocks, where the chalk has been crushed, the striations assume various directions and curvatures, according, I believe, to the direction of the crushing force. Each surface of the large blocks has one uniform line of striation, though the other sides may be perpendicularly, horizontally, or diagonally striated, as if each block had been moved in various directions. The surfaces of the blocks are more or less embossed; but the striæ enter into all the irregularities, and it often happens that a detached piece of rock lies crushed into a hollow, like a dab of mortar, yet both its external surface and that of the cavity in which it lies are striated by lines in continuity with those of the rest of the surface. Mr. Fordham doubts the existence of horizontal striation; but since his paper was read I have had the pleasure of showing him these phenomena, which have shaken his previous opinion, if not convinced him of the correctness of the slickensides theory. The great difficulty I meet with is to account for the sudden steps or breaks, which give to the surface the fibrous appearance of petrified wood, described by Mantell, but which he attributed to its true cause. In this way I account for the fibrous form, by the disturbance having acted only at short distances; for it is not on the large surfaces of the blocks that it is found, but in the angles between the blocks, where pieces detached from the adjoining blocks have been crushed into the angles to fill up the vacancy when the blocks settled down again into their former places.

My attention was first called to this subject by finding an Echinoderm imbedded in a fissure, with its exposed surface striated; and soon afterwards I found others crushed, with taluses formed behind their tubercles in the line of pressure. With such evidences of movement in the chalk I began to examine the fissure-, and rocksurfaces; and I found not only striations and polished surfaces, but calcareous spar, true slickensides, filling up fissures in the chalk, marl, and greensand. I have found exactly the same appearances in the chalk of Brighton and Lewes; and having examined various rocks (mountain-limestone, coal, serpentine, iron, red sandstone, slate, and surturbrand) and found similar striations, I think it illoQ. J. G. S. No. 121.

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