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variety or other of hornblende, of diallage, and occasionally perhaps of other minerals, which (as hypersthene) enter more sparingly into the composition of rock masses. Such admixtures can be properly studied only in those endless suites of specimens which nature herself preserves, and presents in situ. The subject is an interesting one, and well deserves closer attention and investigation than it has yet met with. The general character of the gneiss in question, the brownish purple colour by which it is almost every where distinguishable, its very limited extent, and its gradual passage into the common killas, have been sufficiently noticed by most recent observers. I do not recollect ever to have seen any mineral substances imbedded in its mass. It is of course traversed, like the adjacent rocks, by metallic and other veins. This passes, as is well known, by a rapid transition into the common killas or clayslate. Before entering on the consideration of this, by far the predominant form of stratified rock throughout the whole of the west, it may be well to notice two of its subordinate beds, or, perhaps, varieties, which, though much inferior in point of extent, yet present appearances much more clearly indicative of their mineral composition, and capable, perhaps, of throwing some light on that of the common killas with which they are so closely associated. 1. Common chlorite slate. This needs no further description, and I am not aware that it has ever been found to contain any other imbedded mineral than the garnet, specks of iron, and perhaps of copper pyrites. 2. A laminated rock, of a silvery grey colour, and micaceous aspect, exhibiting throughout its mass small patches of a darker tint, having the appearance of some imbedded mineral obscurely crystallized, and much intermixed with the slate containing it. These patches have been regarded as allied to grenatite, to hornblende, and to some other mineral species, but closer examination shows them (unless I be mistaken) to consist of a dark-grey chlorite minutely and confusedly crystallized. This variety of killas contains in the neighbourhood of Camelford, where it may perhaps be most advantageously studied, small contemporaneous veins of crystalline felspar; and in one or two instances alternates with thin beds of compact felspar tinged by the admixture probably of chlorite; a circumstance observable also in the killas of Wheal Maudlin; and in that which succeeds the granite near Ivy Bridge; though, in these atter cases, it is possible that the penetrating matter may be hornblende.*
But these varieties are, as I have stated, but of partial occurrence and limited extent. The stratified rock of the mining district is almost universally the common killas. This rock, after much question (which your readers would scarcely wish to
* I may here observe that the neighbourhood of Ivy Bridge offered by far the most beautiful and characteristic specimens of compact felspar unaltered apparently by any mixture, which I ever met with in the west.
be recapitulated), as to its being a variety of greywacke, which, if that term has any definite meaning, it unquestionably is not, has been at last admitted on all hands to be genuine clayslate. But this appellation perhaps, after all, does not convey a much clearer notion of the real nature and constitution of the rocks included under it than the repudiated greywacke. An opinion on this subject (nearly identical with that which has for many years been my own) is to be found in the very interesting Catalogue Mineralogique of the Comte de Bournon: "Les parties composantes qui entrent dans la substance du killas sont, le mica vert tres attenué nommée chlorite, le quartz, et le feldspath; et les varietes qu'il presente dependent de la manière dont ces trois parties se réunissent entr'elles." (Bournon Cat. Min. 463.) Mr. Hawkins, in a paper written evidently without the knowledge of C. Bournon's work (probably indeed from materials collected before its publication), appears to hold nearly the same view. "There is much reason (he writes) to consider it (killas) as an intimate mixture of quartz with mica, talc, chlorite, and perhaps, in some instances, with felspar. We may trace the last in those varieties of the slate which, in this country, are contiguous to the granite.* On the other hand, the talcose ingredient of this mixture is more conspicuous in the varieties which occur at a distance from that rock." that, which I cannot but suspect to obtain as a general law (see Annals, vol. v. p. 189); namely, that stratified rocks are in their mineralogical composition only varieties of the crystalline masses with which they are most largely and closely associated, be admitted, we shall have difficulty in recognising in the numerous elvans by which it is traversed, the crystalline analogue of the killas. The substance occasionally termed claystone,† might perhaps afford a link in the series connecting the two extremes. At all events, that term, as well as clayslate, has been very vaguely applied, and is in itself ambiguous. A ready means of detecting the mineralogical constituents of these and the like obscure aggregates (if we admit them to be such), would be among the most valuable services which chemistry could render to geology.
It may be added that the inferior slate occasionally exhibits very remarkable instances of curvature and contortion. coast of St. Agnes, a spot highly interesting both for the mineralogist and geologist, will afford more than one example.
* This restriction is not universally borne out by facts: it should rather have been stated, that "we may trace the last more abundantly." The passages connected with, and following those which I have adduced, well deserve the attention of the geologist. There are some statements in the preceding half of the same paper which Mr. Hawkins himself, on recurring to the advances made in geological science since the period at which his materials appear chiefly to have been collected, would probably be the first to cancel or to modify. See especially p. 6.
+ A rock of this character is found associated with the chloritic, though it seems more common in the amphibolic series.
These, however, are by no means so numerous or striking as those afforded by the greywacke of North Devon, a circumstance apparently adverse to the theory which would attribute these singular configurations to the agency of heat; for we might certainly expect that the killas, which is easily affected by that agent, near as it is to the central granite, and traversed in all directions by various dykes and veins, would have exhibited more frequent traces of this nature than the refractory and unbending sandstones. But this is a question of mere hypothesis. This portion of the inferior slate does not (so far as my knowledge extends) contain any imbedded minerals; near Camelford, and at some other spots, I have observed in it small contemporaneous veins or nests (vugs, as they are provincially termed) of crystallized felspar and chlorite. Most of its varieties are readily fusible.
(To be continued.)
On the Crystalline Forms of Artificial Salts.
(Continued from vol. v. p. 452.)
In my last communication I noticed the irregularity that frequently occurs in the forms of crystals, whether natural or produced by art, occasioned by an enlargement of some of the planes, and a consequent comparative diminution of others. This irregular character may be said to be almost general, and very frequently might lead to an erroneous determination of the true forms of crystals, if we do not attend sufficiently to the positions of their planes, to their cleavages, and to the measurements of their angles. Another circumstance will also tend to mislead us with regard to the forms of crystals, when compared with the drawings by which they are represented: this is the manner of their attachment to the mass to which they are united; sometimes they are attached by a lateral edge or plane of the figure exhibited in the drawing, and sometimes by the upper summit; in which latter case, the crystal would appear to be inverted, and the order of the lateral planes of several of the classes of prisms, when observed from left to right, would be reversed.
The measurement of corresponding planes on different crystals will frequently differ more than half a degree, and may occasion a difficulty in determining particular planes by measurement, when they meet at nearly the same angle. The angles given here are generally the mean of a considerable number of measurements.
Acetate of Soda.
The primary form deduced from cleavage is an oblique rhombic prism, the cleavages being parallel to the planes P, M, and M', of the annexed figure. Some or all of the secondary planes on that figure occur on many of the crystals. On some crystals only the planes k, or k and f, accompany the primary planes, and on others only the planes a and g, with the addition sometimes of the planes h.
All the planes except ƒ have been measured by the reflective goniometer.
Acetate of Zinc.
The crystals are very thin, flexible, and soft, and fissile parallel to P, but do not afford any other measurable cleavage planes. The primary form indicated by the natural planes of the crystals is an oblique rhombic prism, measuring as follows:
The primary form developed by cleavage is an oblique rhombic prism, the cleavages being parallel to the planes P, M, and M', of the subjoined figure; the secondary planes c and g are the only ones I have observed on the crystals, which are sometimes produced in pairs, and united by the planes c, in such a manner as to exhibit a second entire plane P, joined by an acute angle to the lower acute angle of that which
is exhibited in front of the figure, but inverted in its position so as to be terminated at its lower extremity by the planes and ..
The planes M and M' are generally curved, and the cleavage planes parallel to these partake also of the same character.
Sulphate of Magnesia.
The primary form of this substance has been given by the Abbé Haüy as a right prism with a square base. But from the measurement of several crystals, and from the character of the secondary forms of some of those, the primary may be regarded as a right prism with a rhombic base, whose angles are 90° 30′ and 89° 30'.
I have found only one cleavage, which is parallel to the short diagonal of the prism, and consequently to the plane h of the accompanying figures.
Fig. 1 represents a crystal of a form which frequently occurs, and of which the following are the measurements:
Fig. 2 represents a form under which the crystals also frequently appear. In this form, only two of the four planes e are seen on each summit, and alternating in position as shown in the figure.
On some of the crystals, however, which resemble this figure, the two other planes e may be perceived, but they are very minute.
Tartrate of Potash and Antimony-Emetic Tartar.
The general character of the crystals of this compound is that of an octahedron with a rhombic base. I cannot discover more than one distinct cleavage, which is parallel to the plane a of the accompanying figure.
The following are the nearest to coinciding measurements taken on several crystals: