Sulphate of Barytes
Not only very different substances are found in different localities, but the same substances differ so essentially, that some attention and practice are required to recognize them; and when we cannot have the specimen, a figure conveys the best information : thus I thought it blameable to leave the present unnoticed, especially as it is what for many years was not at all expected in Cornwall. It was found in the United mines in Gwennap in that county, I believe in 1815: the very neat specimen figured at the top of the plate was lent me by the friendly and penetrating Mr. Brooke, whose attention to Mineralogy we shall shew is valuable and conspicuous. I add to t he plate part of a specimen from Glenmalur in the county of Wicklow, which in colour and crystallization bears a remarkable resemblance to it: this was sent me in 1813 by Mr. J. Moore.
The other varieties of Sulphate of Barytes are tolerably distinct according to their localities and the formation they belong to: thus those of Cumberland are generally tabular, that of Bletclungley prismatic, similar to some which are found in Auvergne, and the present intermediate between them. The pretty somewhat stellated manner in which the crystals are placed, gives us some idea of the Gypsum found in Clay; but this is placed on Quartz, ami when the crystals are examined they are soon found to be modifications of Sulphate of Barytes.
Fig. 1 and 2 represent the Crystals upon the Cornish specimen with one or two modifications marked in dots; the terminal faces are primitive. Fig. 3 exhibits the manner in which the groups are formed on the Irish specimen.
Fig, 4 is a kind of made formed of two crystals, joined side to side; the line of union is strongly marked in the center of the terminal face: many such are found in the masses of prismatic Sulphate of Barytes of Nutfield, 1 have a group of several small ones sticking upon the side of a large prism. Fig. 6 presents, at one view, all the modifications I have met with among a great number of crystals from Nutfield: I consider that such an outline is sufficient to explain them, and more easily understood than a number of diagrams with the modifications separate, which I promised in Volume V. page 48, as their relative situations are clearly pointed out, and it is pleasant because it is concise. The faces marked with letters have been described by Haüy, and his letters are used. The faces indicated by figures have not been observed by him: of these, figure 1 is a very conspicuous face; it is produced by a decrease of one row of molecules from the obtuse solid angle of the primitive: the other numbered faces are generally too small and dull to measure. The tabular varieties have generally the faces P. r. d. and o. most enlarged; such is the outline of the bottom of tab. 72, and there are but few modifications to be found upon them that are not shown in this outline.