parallel to the junction-face of the rock; and I regard it, like that in the Karak-Clews gabbro, as a pressure rather than a flow structure.[1] On applying the two Nicols, the major part of the slide is seen to consist of the usual bluish-grey fibrous mineral; and the only difference between that which makes up the mass of the rock and that in the pale golden band is the greater regularity of crystallization in the latter. A few minute crystals show a more brilliant tint.
Carn Sparnack (no. 11).—Obviously an altered olivine rock, though but little of the olivine remains unchanged; the general aspect of the groundmass resembles no. 3, except that there is no tendency to a parallel structure, the irregular network of fine serpentine strings with the granules bordered within by iron peroxide being very distinct. The olive-green mineral, with submetallic lustre, which forms the green marbling, is all or almost all enstatite, though here and there probably is a little diallage; but alterations and staining make it difficult to be sure.
Blade Serpentine from near Cadgwith (no. 10).—The ground of the slide is a pale green serpentine, traversed by numerous strings forming the usual network; these are colourless, except that they are often darkened by the abundant deposit of fine magnetite dust. Several small grains of picotite occur, and a good many crystals of rounded exterior and platy structure—the same which is so conspicuous to the eye by its bright metallic lustre. The rock is clearly an altered olivine rock; and on applying polarized light we see that a few grains of that mineral yet remain unchanged. The metallic mineral is all or almost all enstatite, though part of it is rather altered, and converted into a kind of serpentine; and on comparing some of the specimens with that which I have described in lherzolite from Sem (Ariége), one cannot fail to be struck with the correspondence.[2]
The Balk Serpentine (no. 9, two varieties).—These have the same general character as the Carn-Sparnack rock, both being mottled with olive-green; but the ground of the one is more or less red, of the other rather pale claret-colour. A detailed description is needless; both obviously are altered olivine rock, the chief distinction being that in the former the iron occurs rather in little patches and lines, often fringing the included grains (which are clear but isotropic, and so serpentine also), and is the red peroxide; while in the other it is more generally disseminated, giving a dusty look to the slide, and appears to be mostly the black oxide, while the grains are generally a dull pale green: both contain some picotite. The green mineral
- ↑ It is obvious that very similar structures may result from tension and pressure; thus the fissile structure of certain igneous rocks (as I have shown in Quart. Journ. Greol. Soc. xxxii. p. 140) is like true cleavage. Professor J. D, Forbes regarded the banded structure of glacier-ice as the result of differential motion, like that of slags. Professor Tyndall considers it a pressure structure; while in some igneous rocks a pseudo-foliation seems to result, now from pressure, now from tension.
- ↑ See Mr. Hudleston's analysis of this rock, p. 925.
