Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1056

926 calculated as an oxide. Sterry Hunt had previously detected it in the serpentines of Cornwall, Banffshire, and the Vosges, but states that none exists in the ophiolites of the Laurentian rocks of Canada.

Red Serpentine from the Balk quarry.—Dull purple, ground streaked with green, and containing crystals somewhat similar to the last, but of a green colour and more blended with the general mass. Sp. gr. 2⋅561.

Though so different in appearance, this rock is in composition singularly like the preceding one.

The undecomposed silicate is less in amount, and the water is greater (13⋅28); the relative proportion of silica and magnesia about the same; but there is only 1⋅37 per cent, of ferrous oxide, the total iron remaining nearly the same. The bulk of the ferric oxide occurs probably as hæmatite, dispersed in fine powder through the mass, just like the magnetite in the last example ; in this case the powder does not adhere to the magnet. There is about the same quantity of nickel, rather more alumina; but, on the whole, we have every reason to suppose that we see here a rock whose original composition was very close to that of the black rock. The gradual oxidation of the magnetite into hæmatite, and a slightly more complete hydration of the original basic minerals, constitute the real difference between them.

The Brass-yellow Crystals.—These lose 9 per cent, in ignition (dried at 100° C), and, although composed principally of silica and magnesia, contain more alumina and lime than the matrix, and much less iron. Their chemical composition is similar to that of bastite; and they are probably the result of the hydration of a variety of enstatite.

The following are very rough analyses of some of the rocks associated with the serpentines, and which, as before stated, present such a remarkable contrast to them in the amount of lime and alumina which they contain. This is due to the presence, in all or nearly all of them, of a kind of alumina-lime felspar (?), whose varieties may be grouped under the general term "Saussurite." The amount of alkali in those specimens examined appears to be so small that it has been queried in some cases when making up the total constituents.

One of the most interesting rocks is a peculiar gabbro occurring near Caerleon Cave. This gabbro is remarkable for crystals of diallage of great size, associated with an opaque white mineral mass,