Geological Magazine (Volume 24) (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1887. Excerpt: ... Now I find in all sections of the Bill serpentine that I have examined irregular grains of a colourless mineral having the refractive index of felspar, so far as one can judge of this by the relief of the section and the character of its surface. This mineral always polarizes in the neutral tints of the first order in sections in which olivine and augite would give as a rule chromatic polarization. It frequently shows, moreover, a fine lamellar twinning, and sometimes two sets of parallel lamellse may be seen intersecting each other at a high angle. It has been rendered turbid in places by granular decomposition products. Now I know of no mineral except felspar which possesses all these characters. Professor Bonney vaguely suggests that it is augite or diallage. I am, of course, aware that augite does show multiple twinning; but I cannot possibly regard this mineral as augite. In one case in which the extinction of the two sets of twin lamellae were approximately symmetrical with reference to the trace of the face of composition, the combined angle was 58. Now, if the crystal were augite, twinned according to the ordinary law, such a section could not possibly be cut approximately at right angles to an optic axis;1 and, therefore, in slides of the thickness used, could not possibly polarize in the neutral tints of the first order, as it actually does. Another very important point is the existence of two seta of lamellae intersecting at a high angle. This is perfectly easy to understand on the assumption that the mineral is felspar; but, so far as I know, inexplicable on the assumption that it is augite. To sum up. As the mineral possesses the refractive power of felspar, the double-refraction of felspar, the twinning of felspar, and the mode of alteration...