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The place ot the amethyst in systematic mineralogy :»» cheJcal constituents, crystallographic form, the characters of the species ot which it is a variety, its colour geognostic situations, and geographic distribution are so -'^^l ^no™ ^^ scarcely to call for remark. There is still some difference of opinion as to it constituents, traceable no doubt to the fact, that these are not constant, but vary in different specimens Eose's analysis, which is that most generally received is as follows: sUica 97-50, alumina 075, iron ox.de and traes of manganese 0-75. In a Brazilian specimen, Hemtz (quoted by Dana) found traces of magnesia and soda, whose presence he thinks accounts for the characteristic colour of th^ mineral Others hold this to be due to a small P— f °f oxide of manganese. Amethyst occurs ni veins or linmg the oft-described agate balls. "Crystals withm the g odes or hollow agate balls are very often of an amethyst o our, and some are very fine" (Cronstedf . "Mmeralogy, vol. .p. 151 17S8). I am able to show to the Society, a very bea„t,f„i ™up of pure amethyst crystals in an agate ball from Saxly The gem known as oriental amethyst is spmel or «:i*a; c^ru^nn.,. widely different minera wi^h an amethystine hue. The constituents ot spinel are ali.mm. Hm 74-50, silica 15o6, magnesia 8-25, oxide of iron 156, lime 0-75. The variety of amethyst which forms the subject of this notice was presented to me, without any reference whatever to its scientific interest, but simply as an ornamental stone, by the Rev. Dr Paterson, New Glasgow, Nova Scotia, who had obtained it at Prince Arthur's Landing, on the north shore of Lake Superior, in August 1878. Looking at it with a good lens I remarked to Dr Paterson, the specimen is altogether unlike any I have seen. The dark red crystalline substance on the faces of the hexagonal pyramids is not deposited in a liomog'^neous layer, but seems to coudist of innumerable spots, I should say of iron oxide. As the donor thought it must have been described, I consulted most of the leading authorities, without, however, finding any reference to this variety. In a note dated August 2, 1879, Dr Paterson says, "I find that the amethyst from Lake Superior has been analysed by Sterry Hunt of the Canadian Geological Survey, who discovered the colouring matter to be oxide of iron before i/ov." On being asked for a reference, he informed me in a subsequent note, tha^ having failed to find it, he hcd written to Principal Dawson, Montreal, for information. Principal Dawson, writing in the absence of Dr Hunt from town, on November 5, 1879, says — "I write now merely to state what I know as to the matter referred to. The ferruginous coating which you mention is very common on crystals of amethyst from Thunder Bay, and seems to have been simj y the latest coat of quartz deposited on the crystals, and containing peroxide of iron in little rounded hollow concretions with radiating spicules. This mode of arrangement of oxide of iron is not unusual in reddish agates from Nova Scotia and elsewhere, though with various modifications in detail. I am not aware that it has been par- ticularly described, nor that any special cause of it is known further than the general one of molecular and crystalline aggregation, which has to do duty in the explanation of an infinity of curious forms in agates and other forms of quartz. I cannot find that Dr Hunt has published any particular account of the peculiar appearance in the Thunder Bay ^i \ * ■¥{ 3 amethysts." A thin slice prepared for magnified ninety diameters, presented shown in the somewhat coarse, but plate which accompanies this notice, fessor Grum Brown kindly took charge he entrusted to Dr Gibson for analysis, specimen as follows : the microscope, and an appearance well highly characteristic At my request Pro- of a fragment which who reported on the " University of Edinburgh, 3farch 27th, 1879. ** Report lijpon Crystal of Amethyst Quartz. "A qualitf^tive examination was made with a view to determine tho nature of the red colouring matter deposited underneath the surface of the crystal. The result of this examination showed the presence of iron, and the absence of copper and other heavy metals. Ferric oxide being of very common occurrence in quartz, there is no reason to doubt that it is the red colouring matter in the crystal examined. "J. Gibson, Ph.D." My first impressions as to the colouring matter of the six- sided crystals of this specimen were thus verified. It might, however, be well to carry the analysis farther by testing for soda, magnesia, or manganese, having previously marked the degree of intensity of the violet blue colour of the specimen. But, apart from this, it will be seen from the pieces in the lump and in section now exhibited, that they are made up as follows : The base on which the crystals rest is a thin layer of fine vesicular trap. Above this is a mass of highly crystalline semi-transparent quartz, about an inch in thickness, thickly packed but yet showing the planes of the crystals less or more well marked, and, on the top of this a thin layer, of granular-like amorphous quartz, out of which the definitely crystallised amethyst proper seems to rise. This may or may not be generally the order of the layers, but in the specimens now before us it is well marked. The dirty red colouring matter is confined to the faces of t) hexagonal pyramids — the characteristic crystalline forms of quartz — and is, for the most part, deposited in pretty separate annuli, ring within ring. In the specimens now under notice I have not seen any traces I of the " radiating spicules," referred to by Dr Dawson. Nor are the rings on the same plane. When examined through a good binocular they are seen to lie at different levels, a fact which seems to warrant the inference that the highly crystalline glaze, so to speak, in which they lie, consists of different layers. The spots are not in all cases perfectly circular, as may be seen by referring to the accompanying plate. It would not be profitable to speculate on the probable explanation of those molecular aggregations. I may, however, ask the Society to look at the so-called morpholites or clay concretions, and the dolomites from Cumberland, now on the table, as illustrating, on a large scale indeed, in a somewhat striking way, the close resemblance between them in point of form and the spots figured on the plate. This resemblance suggests a topic of great interest anu of whica little has yet been made. I refer to the analogies between the power of concretion and that of crystallisation. But 7 do not wish to make more of this resemblance than to indicate the fact. The crystalline matrix in which the spots occur has, for want of a better term, been called a glaze. Is tl o presence of this necessary in order to the iron oxide arranging itself in such spots ? In the New College Museum is a large lump of rock crystal, on which the faces of the six-sided pyramid are covered with a layer of iron oxide, lying wholly on the surface, in tlie form of rough amorphous particles, and destitute of this glaze. As I have been unable to find any published description of this variety of amethyst, and as it presents some features of consideralile interest, I have thought it not unworthy the attention of the Society. The plate is an attempt to re- produce, in a rough way, the colours and the forms of some of the spots shown in the section, exhibited to the Society under the microscope. V 1 \ M'Farlane d: Erskine, Printers, EdivMirgh. / f ^ mp..i>! | i» i H | i