Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1037

Rh red variety; for towards the outside it exhibits perfectly the change into the red variety. I have specimens about 4 inches long, red at one end and green at the other. Hence the alteration may not be so deep-seated as from its uniformity I should have supposed.

I have examined three slides of the red, cut from slightly different varieties of the rock, and one of the green. I will refer to them by numbers, taking the most normal specimen first.

On placing slide I. beneath the microscope, we find it to consist of colourless or nearly colourless felspar, in rather irregular to roundish oblong grains, occasionally showing lines indicative of twinning, traversed often by cracks and in places partly kaolinized. Associated with this, in about equal quantities, are a large number of irregular grains of olivine; these in parts are almost unaltered, in other parts entirely converted into serpentine. This is occasionally translucent and of a greenish yellow, occasionally opaque, from the presence of a muddy brown peroxide of iron, and showing every grade of intermediate staining. The process of conversion of the olivine into serpentine will be described below.

With polarized light (crossed prisms) the felspar is seen to be crystallized in irregular grains, many of which show characteristic plagioclase twinning; bright colours, however, are rare, shades of light and dark milky grey being commonest. In parts the felspar is almost opaque from decomposition; in other parts it presents the usual finely granular "saussuritic" aspect. The olivine, when unchanged, shows its characteristic rich colouring. The process of conversion into serpentine, best examined by rotating the polarizer, is as follows (see figs. 8 & 9). In the cracks of the olivine a dark ferruginous stain is deposited; then on either side of this a layer of fibrous serpentine of pale golden colour (probably chrysotile) is formed; thus the olivine grains seem traversed by an irregular network of associated dark and light strings. The interspaces then seem to be attacked; and they also are converted into serpentine; but in them the mineral is usually in an isotropic or noncrystalline state, and the peroxide of iron either forms a dark clot in the middle