During the last few years some fresh specimens of Sparable Tin have been raised from various mines in Cornwall; a description of this variety may therefore be of interest to the Society.

The value of this description of the now finds will perhaps be increased by combining with it an account of the specimens of Sparable Tin from different mines now to be found in our museums and private collections. Through the kindness of those gentlemen who own or have charge of these collections, I have been enabled to see and examine many of the interesting specimens which they contain.

The name "Sparable Tin" is peculiar to Cornwall, and has been given to this variety of Cassiterito on account of its so-called resemblance to the sparable nail of the cobbler.

The subject of this note derives interest less from its novelty than from the beautiful manner in which the intergrowth is exemplified in Nature. As Sadebeek has shown, the intergrowth of marcasite and pyrites takes place according to two laws.

(1) The vertical axis of marcasite is parallel with a principal axis of pyrites; one of the two other principal axes of the latter being parallel with the combination edge m (110) ∞P : c (001) 0P of the former ; or, in other words, the vertical and one lateral axis (ab) of marcasite coincide with two principal axes of pyrites.

(2) The vertical axis of marcasite is parallel with a principal axis of pyrites; the braehydiagonal axis is parallel with a diagonal axis of pyrites.

Although all the modes of hemihedrism possible in the cubic system have been found to occur among minerals, no mineral l~elonging to that system has yet been proved to be tetartohedral like sodium chlorate, and the nitrates of barium, strontium and lead.

There has, however, been some evidence in favour of the tetartohedral character of Ullmannite since the examination of the crystals from Lölling in Carinthia, and those from Sarrabus in Sardinia. The former were found by Zepharovieh to be distinctly tetrahedral in form and habit, the latter were found by Klein to be distinctly pyritohedral. Zepharorich found the specific gravity of the Lölling Ullmannite to be 6·72, while Klein found that of the Sardinian crystals to be 6·803-6·883, and for this reason it was at first thought that the two occurrences might be different modifications of the antimono-sulphide of nickel; there was also considerable discrepancy between the two analyses.

This instrument, which was designed for use in the Crystallographic Laboratory of the City and Guilds of London Institute (Central Institution), is in its simplest form (Plate III.) a strong and inexpensive Wollaston Goniometer. It is, however, constructed in such a way that it may be converted into an instrument of greater precision (Plates IV. V.) by the simple addition of accessory parts to tho apparatus of Plate III. without any previous alteration.

L is the graduated disc, and B the milled head by which it is turned; the vernier v is not the slender tongue of metal which is usual in such instruments, but is engraved upon a portion of the fixed disc V; and L is kept in continual contact with V by means of a circular steel spring inserted between B and the bearing of the axle.

The existence in the British Isles of a rock essentially composed of garnet and omphacite, and answering therefore to Hauy's' definition of eclogite, does not appear to have been hitherto recorded.

The following description of such a rock from Totig Ferry, Loch Dutch, is therefore of some interest. The specimen was collected by the Director General, who informs me that he believes it to be a part of the Archæan gneiss which in the region of Loch Alsh and Loch Carron has been thrust bodily from the east over the Torridon sandstone.

The rock is of a dull green eolour, and contains numerous reddishbrown garnets. Under the microscope the principal constituents are seen to be garnet and omphacite—a pale green pyroxene without any definite crystalline form. Green hornblende, plagioclase, rutile, ironores, quartz, and epidote occur as accessories.

Riebeckite was first described by Sauer in 1888. The type-mineral occurs in granite from Socotra collected by Dr. E. Riebeck.

In the same year Harker identified the peculiar blue hornblende occurring in the quartz-porphyry of Mynydd Mawr with the mineral described by Sauer, and in the following year two fresh localities were given for this mineral by Le Verrier and Lacroix. The last mentioned observer has also shown that croeldolite must be referred to the same group of hornblendes.

The object of this communication is to place on record the occurrence of riebeckite in the so-called syenite of Ailsa Craig, the small island which stands at the entrance to the Firth of Clyde.

In the summer of 1890, in the company of Mr. L. W. Fulcher, I again visited Mynydd Mawr, on the west of the Snowdon area, in the hope of obtaining specimens of Riebeckite larger than those on which Mr. Harker and Professor Bonney based their observations in 1888, when they independently pointed out the occurrence of this mineral for the first time in the British Isles. Previous traverses of the dome-shaped mass had shown how uniform in structure and how minutely crystalline the rock of Mynydd Mawr was over all its surface; we accordingly examined the columnar cliffs on the north and west, paying especial attention to the great western hollow, where denudation allows one to stand almost in the centre of the intrusive neck. But even here we were somewhat disappoinied, and our specimens are only slightly coarser in grain than those studied with such striking results by Professor Bonney.

The process to be described has been applied to the minerals mentioned below, but I have no doubt that all similarly constituted minerals will also be easily recognised by the method.

I am quite sure that all mineralogists have experienced much annoyance and difficulty in obtaining really satisfactory results with the ordihary blowpipe reactions in regard to some of the compounds of arsenic, antimony, and sulphur with metals, particularly in the separation of the metal or metals from the above-mentioned elements. In order to obtain the metal the assay requires a most careful and tedious roasting on charcoal in the oxidising flame of the blow-pipe, in order to volatilise away the sulphur, arsenic, or antimony. When this roasting is complete the residue (oxide) is mixed with either sodium carbonate, or that salt and cyanide of potas. slum, and fused for a considerable time on charcoal in the reducing flame in order to obtain a bead of the metal.

In this paper the author described his method of constructing crystal models of paper or cardboard by means of plaiting. In the ordinary crystal "nets" (such as, according to Mr. Gotham, were constructed by himself and shown at the Royal Society as early as 1847) the faces of the model are drawn in juxtaposition on cardboard and cut out, some of the lines being only partially cut through ; the net is then folded up so as to form a solid figure, and the faces are united at their edges by gum.

In the models described and exhibited by Mr. Gotham, the faces are so drawn as to be disposed along strips; these strips are united to each other at one end, and can then be folded together by a process resembling plaiting, in which the strips pass alternately under and over each other.