dation in the motions of the air rushing in from the colder to the warmer surface below is also produced from the resistance of the air lying over the latter. The production of sea and land breezes furnishes a complete and instructive illustration of these remarks. Many of the upward currents, which I observed with the anemoscope during the summer mornings, were undoubtedly the precursors of the sea breeze. Such currents continue to accompany the production of the land and sea winds in a manner that I have been able sometimes to observe by the smoke of steam vessels near the coast. Thus, on a warm day in June, 1857, I observed the simultaneous existence of the sea breeze at Kingstown and a slight motion of a few light clouds from the interior towards the coast. A steam-ship far out at sea was proceeding towards England, and the smoke was drawn by the gentle breeze into a streamer extending for miles behind the boat. The streamer of smoke appeared straight and perfectly horizontal over the surface of the water, until it arrived at a point about a quarter of a mile from the Hill of Howth, when it rose upwards with a gracefullycurved outline, and it appeared to be gradually diffused in the air situated vertically over the hill.

The influence of vertical and oblique currents in the atmosphere is not only thus manifest in the comparatively limited and local phenomena of sea and land breezes, mountain winds and whirlwinds, but it has been also appealed to in order to explain the circulation of the great winds of the Earth. Thus Maury, in his attempt to exhibit the general laws of the great winds, presents a diagram in which ascending and descending currents are distinctly indicated over different regions of the globe. Their agency is also appealed to by other inquirers, and their principal seats of action seem to be indicated as the calm regions, that is to say, the regions where horizontal winds blow with least intensity. Observations with the aid of the anemoscope in the regions of equatorial and tropical calms, would thus probably serve to test the accuracy of the general views here alluded to. The systematic study of the non-horizontal movements of the atmosphere has scarcely been commenced, but what little knowledge we possess of such movements shows that they are so closely connected with some of the most important phenomena of the weather, that their further investigation is certain to be attended with interesting and valuable results.

ART. VII-Note on some Prismatic Forms of Calcite from Luganure, county of Wicklow. By WILLIAM K. SULLIVAN.

N the first edition of his Traité de Minéralogie (Paris, 1801) I Haiy distinguished three kinds of prismatic carbonate of lime: 1. Chaux carbonatée prismée, already described by Romé de Lisle, and which Hauy supposes to be derived, in his molecular theory of decrements by the law d'. According to this, it would be the prism produced by modifying planes placed upon the lateral edges of the primitive rhombohedron. The second he calls chaux carbonatée imitative, and considers to be the prism obtained according to the law e by planes on the lateral angles of the primitive. The third, which had also been before described by De Lisle, he named chaux carbonatée prismatique, and considered to be also derived according to the law e2. He mentions four varieties of this form: a, alternating-having three alternate wide faces and three intermediate narrow ones; b, compressed-with two opposite faces larger than the other four; c, widened-with four faces wider than the remaining two; and d, lamelliform-in very short (i.e. in tabular) prisms. Of the crystals of this form he says: "In certain crystals the extremities are of a dull white, while the intermediate part is transparent. In others the opaque part is situated towards the axis and surrounded by a transparent envelope. The bases of a few exhibited concentric hexagons, and one could even observe the extremity of a small internal prism, rising above the whole prism".

The forms he calls imitative and prismatic being obtained by the law e2, contain the same prism; the prismatic faces which have been observed among the varieties of calcite belong, therefore, to one or other of those prisms. Dufrenoy, who uses the nomenclature of Haiy, as modified by Levy and himself, represents the faces of the first prism, or that on the edges of the rhombohedron, by the symbol d1 (u of Haüy), and the prismatic, or that on the angles, by e2 (c of Haiy). Of course each of these prisms is completed by the modification at on the summit angle, which produces the horizontal plane forming the base.

According to the German crystallographic methods, prisms are looked upon as mere limiting forms. Mohs and Haidinger consider d to be the limiting form of the pyramids, the former expressing it by the symbol P+ and the latter by P, which is the one adopted by Zippe in his summary of all the observed

forms of carbonate of lime.1 The second prism e is considered to be the limiting form of the rhombohedron, and is represented by Mohs by the symbol R+∞, and by Haidinger by ∞R. Zippe also adopts the latter.

According to Haüy d' or ∞P is rare, and Dufrenoy states that only some examples are known. According to Zippe, it is frequent enough in combination as a secondary form, but seldomer as the dominant form. Surmounted by the primitive rhombohedron (R or P), it is noticed by Dufrenoy as a very rare example of the prism on the edges, associated with the primitive rhombohedron" from Cumberland. He also mentions another in which b' or R' (the equiaxe of Haüy) replaces P or R, but does not give the locality. Further on he notices a third example from the Samson mine in the Hartz, in which the horizontal edges of the prism are truncated by rudimentary planes of the pyramid. The prism R or e, although comparatively rare as a simple form, is very frequent in combination; according to Dufrenoy indeed, it is the only one found complete. A little before, he says that it is of a milky whiteness, and almost always opaque. The base sometimes bears striæ parallel to the edges, which are indications of cleavage. Examples of R surmounted by R' or b' from the Hartz, Cumberland, and the department of l'Isere,

have been described.

The position of the rhombohedrons surmounting the prisms is different in each kind. In P the surmounting rhombohedral faces lie so that the edges of combination with the prismatic faces coincide with the lateral edges of the rhombohedron. In

R the edges of combination in three alternate faces are horizontal; the truncatures at either end of the prism alternating, so that each face of truncature is parallel to one at the opposite end. The directions of the cleavages correspond perfectly with the dispositions of the modifying planes, so that every alternate basal edge of the prism R or e2 may be removed by cleavage with the greatest facility, by which a prism surmounted by the faces of the rhombohedron may be obtained. Although the prismatic faces R are sometimes dull, they always, at least in all the crystals which I recollect to have seen, possess more lustre than the faces P associated with them. The former are, indeed, usually very bright in transparent crystals. This circumstance is noticed by Dufrenoy, who, in speaking of the example of P or (d') with pyramidal trun

1 Uebersicht der Krystallgestallten des rhomboedrischen Kalk-Haloids von F. X. M. Zippe.-Denkschriften der Kaiser. Akademie der Wissenschaften. Mathematisch-naturwissenschaftliche Classe iii. Bd. 1st Lief. p. 109. 'Traité de Minéralogie par A. Dufrenoy. 2me Ed. Tome 2me, p. 297.

III.

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catures of the lateral edges, from Samson mine in the Hartz, says that the faces are dull and somewhat rough, as is frequently the case with those prisms ("les faces en sont mates et un peu raboteuses, circonstance frequent pour le second prism (i. e., d') à six faces"). The difference in lustre between the faces of the two kinds of prisms is characteristically seen in the dodecagonal prisms (chaux carbonatée periododecaédre of Haüy), which is the combination ∞P, R, oP (d' e' a'); the faces R (e) are always very much more brilliant than P (d). This difference of lustre is one of the distinctions relied upon to distinguish the faces of the two kinds of hexagonal prisms from one another. Dufrenoy also notices this difference between the two kinds of prismatic faces in the twelve-sided prisms.

Several forms of the rhombohedral prism occur at the Luganure mines, county of Wicklow, which are worked for galena in a veinstone consisting chiefly of quartz, in a granite country. Among these may be mentioned ∞P, OR (d', a'), consisting of small hexagonal prisms, with very bright prismatic faces. One half of the prism is hyaline, and the other opalescent; the base, OR is dull. Another variety of the same form also occurs, consisting of crystals one centimetre high, and with basal edges one centimetre long. Each crystal has a sort of rude triangular prismatic milky nucleus, surrounded by a perfectly hyaline envelope, reminding one of the description of Haüy given above. Owing to the number of cleavage planes, some crystals are not transparent. The face oR is, in most instances, peculiarly striated, in others it is, as it were, coated with a thin porcelaneous layer. These crystals may be easily cleaved parallel to the alternate basal edges, which are sharp, and without any trace of modifying planes. The form oR, R (a1, e2) also occurs in beautiful hexagonal plates, with very bright prismatic faces, and composed of exceedingly thin alternating layers of white opaque, and hyaline matter, the base oR being always opaque, dull, but beautifully white. Haüy's description of the prismatic kind embraces this variety likewise-in fact, the specimens from Luganure here described illustrate perfectly Hauy's description.

I have lately, however, met with another form, consisting of hexagonal plates, of from one millimetre to one and a half thick, with basal edges of from five to twenty millimetres. The base has a bright nacreous lustre, much brighter than what I have ever seen in any other specimen; striated and uneven, in consequence of the lapping of smaller plates. The most of the tabular prisms are, in fact, compound twins to the base oR (a'). Some twins also occur to the faces of the prism, and finally, to a

rhombohedron. It is owing to this twin structure that the crystals are not generally transparent, for in thin plates they are perfectly hyaline. Except for the difference of form, a mass of these crystals, resting on crystalline quartz, resembles, in a striking manner, a mass of large crystals of chlorate of potash. Layers of growth in the direction of the secondary axes can be observed in some of the prisms; in many of these the outer shell, about one millimetre thick, is frequently free from indications of cleavage, and perfectly transparent. The prismatic faces are dull, exactly like the appearance of white wax, when sufficiently thin to be translucent; they are also uneven. These faces exactly resemble those of the prism P (d'), in specimens which I have seen from Andreasberg. On this account, I concluded, at first sight, that I had the combination oP, P, which would be not merely rare as an example of the pyramidal prism, but still more so as a tabular form of it, in which the base would impress its character upon the crystal, and of which I have not seen any example recorded. I found, however, that the alternate edges were modified by rudimentary facets of a rhombohedron, which was placed in the same position, as regards the faces of the prism, that I have before mentioned as characteristic of R. The basal edges not modified were easily removed by cleavage. I found the modifying facets to be those of the rhombohedron R' or b'.

Associated with the crystals just described, were sometimes found white opaque crystals, like those from Andreasberg, and others three or four millimetres thick, upon which were rudimentary facets of a scalenohedron. I have not been able to get any good specimens of these varieties.

It may be worth while to enumerate, from Zippe's excellent memoir, the tabular prismatic forms which have been hitherto observed; with a view of determining the exact position of the example just described in the series They are as follow: 1. OR, 2P, ∞R, ∞P (a', e3, e2, d) figured by Levy.3

2. OR, R', R (a', b', e2) white tabular crystals from Weardale in Durham.

3. OR, R, ∞P (a', e2, d') from Andreasberg.

4. OR, 2R' coR, P (a', e', e2, d') from Andreasberg.

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5. OR, ‡R', R, P from Andreasberg.

6. OR, R from Andreasberg, Marienberg, Schneeberg, Joachimsthal, and Schemnitz.

The last mentioned form from Luganure, which is oR, ∞R,

Description d'une collection de minéraux formée par H. Heuland, etc. Londres, 1837, fig. 87.