ART. XX.-Description of the Circular Segment Micrometer. Communicated by the Author.

As the circular micrometer has now come into universal use, and has been employed by most of the distinguished conti nental astronomers in all their observations on comets, and in determining the differences of right ascension and declination' of the stars that pass through the field of the telescope, I trust that any suggestion for the improvement upon such an instrument will be acceptable to the astronomical readers of the Edinburgh Journal.

In a late number of the Philosophical Magazine, * Mr. F. Baily has published an excellent paper on Circular Micrometers, and in the same number there has appeared a translation from Professor Schumacher's Astronomische Nachrichten, No. 43 of Frauenhofer's description of the different improvements which he has made upon the instrument. † I shall therefore suppose the reader to be acquainted with what has been previously done upon this subject, and proceed to describe the new micrometer which has been suggested to me by a perusal of these papers, and especially by the remark made by Frauenhofer, "that, for determining the relative place of two very near stars, for instance, a double star, there still remains a good deal to be wished for in the described micrometer."

In place of a complete circle fixed to the diaphragm of the telescope, I propose to use one or more circular segments, as shown in PLATE I. Figs. 7, 8, and 9. In these figures, where AB is the diaphragm, the two segments CmD, CnD, turned and finished in the most accurate manner out of the samé circle, are placed so as to intersect each other, either in the circumference of the diaphragm as in Fig. 7, or at two points within the diaphragm as in Fig. 8, or so as not to intersect at all as in Fig. 9. In all these cases the differences of right ascension and declination of two stars, whether they are yery near each other, like double stars, or at a greater dis

* Philosophical Magazine, March 1824, vol. lxiii. p. 177.
See the Scientific Intelligence of this number.

tance, may be obtained by not very complex formulæ, from their repeated immersions and emersions behind both the con cave or both the convex sides, or behind one of the convex and one of the concave sides.

As the formulæ depend upon the radius of the circular segments, it is, of course, taken for granted that the astronomer can readily determine this element, either from the passage of an equatorial star across a diameter of the circle or by other When the segments are taken from the same circle, one of them may be made fully equal to a semicircumference. of the circle, so that the determination of the radius becomes easy.

means.

When the segments cross one another, as in Figs. 7 and 8, the circular edges may, by nice and flat turning, be brought into the same plane with more accuracy, than two of the smallest wires intersecting each other in the field of view.*

ART. XXI.-On the Passage of Basalt into Granite. By SAMUEL HIBBERT, M.D. F.R.S.E. and Secretary to the Society of Scottish Antiquaries, &c. &c. Communicated by the Author.

THE circumstances under which basalt is found connected with granite, must necessarily form one of the most import ant of geological investigations. This interest has arisen from the alleged igneous origin of basalt having been considered as far less disputable than that which is assigned to granite. For several very valuable communications on this subject, we are, in a particular manner, indebted to Dr. MacCulloch, who, in his notice of many sites of rocks, illus trative of the junctions of basalt and granite, has adverted to the vicinity of Hillswick Ness, in Shetland. In the account which I published of the geology of this country, it appeared more doubtful to me than at present, whether any of its rocks

* Those who are desirous of studying the uses of the Circular Micrometer may consult Baron Zach's Monatliche Correspondenz, vols. xvii. xxiv. and xxvi. where Bessel's formulæ are given for almost every case; Santoni's Elementi di Astronomia, tom. i. p. 261; and Mr, F, Baily, in the Philosophical Magazine, vol. lxiii. Pr 179.

afforded a perfect illustration of so interesting a circumstance; but the different view which I now entertain, arises from subsequent observations which I have made on the nature of trap-rocks in other districts, and from a re-examination of the very extensive collection of specimens that I have brought with me from the Shetland Islands.

It is not difficult to account for the great confusion that has long prevailed on the subject of trap-rocks in general, when we reflect, that the nomenclature of their different varieties depends upon the presence of such mutable ingredients as hornblende or augite, which by almost insensible gradations are so apt to pass into each other, as not unfrequently to possess a sort of intermediate character. The French distinction, therefore, of diabase and dolerite, is too frequently no less difficult to be made than that which is attempted by British geologists, when they speak of greenstone, of augite rocks, or of basalt. Another source of confusion arises from the endeavour to affix certain names to the different degrees of proportion in which hornblende and augite are severally united to felspar, and even upon their more or less intimate states of combination. As far, however, as I have yet been able to collect from the various descriptions which authors have given of what they actually mean by the term Basalt, it seems most generally applied to any intimate commixture of hornblende, or even of augite, with felspar, when the last-mentioned ingredient is found in a much smaller proportion than in that rock to which, in contradistinction, the name of greenstone is given. But, at the same time, the analysis of basalt indicates, that under this designation another variety may even be included, for it would appear, that the small proportion of felspar contained in it does not manifest the presence of potash, like some other varieties of this ingredient, but exclusively soda.

The basalt, which is the subject of the present paper, consisted of hornblende in an intimate state of union with a very small proportion of felspar. It is of a dark green colour, and possesses a structure which is minutely granular. The trap-rocks, of which it forms a part, extend from the island of Mickle Roe northwards to Roeness Voe, a distance of twelve miles, and are here from one to three broad. On the west

of these trap-rocks is a considerable mass of granite, which may be twenty-four miles in the same northerly direction. This granite almost exclusively consists of the ingredients of quartz and felspar. I could find no mica in it.

The last characterized rocks of basalt which occur among the trap series of this district, are to be found in the vicinity of Hillswick Ness and near Mavis Grind, at which latter place they are contiguous to another granitic mass. But at each of these spots, their passage into granite may be observed. This transition I shall now describe. Not far from the junction, we may find interspersed through the basalt very minute particles of quartz. This is the first indication of an approaching change in the nature of the rock. In again tracing it still nearer the granite, we find the particles of quartz dispersed through the basalt becoming still more distinct, more numerous, and larger, an increase of magnitude even extending to every other description of particles. The rock may now be observed to consist of separate ingredients of quartz, hornblende, felspar, and greenstone; the latter substance (greenstone) being a homogeneous commixture of hornblende and felspar. Again, as we approach still nearer the granite, the disseminated portions of greenstone disappear, their place being supplied by an additional quantity of felspar and of quartz. The rock now consists of the three distinct ingredients of felspar, quartz, and hornblende. The last change which takes place results from the still increasing accumulation of quartz and felspar, and from the proportionate diminution of hornblende. The hornblende at length eventually disappears, and we have a well-characterized granite, consisting of the two ingredients of felspar and of quartz.

From this example, then, it would appear, that the transi tion of basalt into granite essentially depends upon the gradual development of quartz. At the same time, there is an increase of felspar, and there may also be a diminution or total disappearance of the hornblende.

Near Hillswick Ness, the passage of felspar-porphy granite may be likewise noticed. Some of the contine: logists who describe the transition, and who speak of porphyry as a " disguised granite," vaguely conceive change is effected by a mere increase of the size of

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ponent particles. But it will be here seen, that the passage of felspar-porphyry into granite, not only depends upon the : particles of the rock which undergoes the transition becoming larger, but (as in the case of basalt,) upon the gradual and distinct development of the ingredients of quartz..

I cannot conclude this account without recommending the examination of the trap-rocks in Shetland to the attentive ex-) amination of the geologist who may be induced to visit this remote part of the British dominions. There is an invasion of the granite to the north of Roeness Hill by a large dyke of trap, that sometimes attains a width of two or three hundred yards. Its ramifications are magnificently displayed in the steep declivities which bound the west coast of Northmavine.

ART. XXII. Description of two surfaces composed of Sili ceous Filaments incapable of reflecting light, and produced; by the fracture of a large crystal of Quartz. By DAVID BREWSTER, LL. D. F. R. S. Lond. and Sec. R. S. Edinb.

THE very remarkable specimen of Quartz which I propose to describe and exhibit to the Society belongs to the cabinet of the Marchioness of Huntly, who was so kind as to leave it with me for particular examination. The original crystal, of which it forms a part, was two inches and a quarter in diameter, and of a light smoky colour, but impervious to the light except in small pieces. It was broken up for the purposes of his pro- ; fession by Mr. Sanderson, lapidary in Edinburgh; who remarked the blackness of the fracture, and laid it aside for the purpose of showing it to me.

At first sight, the absolute blackness of the separated surfaces seemed to me, as it did to every one, to be owing to a thin film of opaque and minutely divided matter that had insinuated itself into a fissure of the crystal; but this opinion was immediately overturned when I observed, that both sur faces were equally and uniformly black, and that they were also perfectly transparent by transmitted light.

Read before the Royal Society of Edinburgh, May 17, 1824.