CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 491 
decay of some other minerals. Where it abounds the titaniferous ore is usually 
greatly oxidised, streaks and diffused grains or blotches of hematite, but more 
frequently of limonite, being abundant. 
Serpentine or some serpentinous product is a frequent constituent of the 
rocks. It occurs of a pale apple-green colour ranging to dirty brownish 
green, is sometimes finely fibrous in tufted or plumose forms, and shows the 
characteristic aggregate polarization. In some rocks, that of Corstorphine for 
example, it occupies interspaces comparable in size to those of some of the 
original minerals. It might in such cases represent former olivine, but, as I have 
already stated, I have never detected it in rocks of this type retaining any 
outward crystalline form. It may sometimes be noticed in threads running 
through fissures, both of the augite and felspar. But the visible change of 
augite into serpentine is not common in any of the rocks which I have 
examined. 
Besides the serpentine there occurs, sometimes in considerable quantity, a 
green translucent substance, occupying spaces between crystals, filling up 
cavities in the crystals themselves, and spreading in frequent filaments through 
minute fissures of this rock. In some instances it assumes beautifully fibrous, 
crested, and vermicular forms. Probably delessite and saponite are both 
included in these green alterations. According to the recent analyses of 
Dr HEpDDLE, the green hydrous minerals of these volcanic rocks do not comprise 
chlorite, which he says is distinctively a mineral of the crystalline schists. 
Plates of brown biotite appear in the more altered rocks, and minute prisms 
of hornblende occur under similar conditions. I have never cbserved any 
epidote. Pyrite is rarely altogether absent. 
Calcedony in minute amygdules and strings is not infrequent among the 
more decayed varieties of rock, but crystalline quartz is the usual form in 
which the free silica has been deposited among the interstices of the rock. 
These quartz patches are distinguished from the original blebs of that mineral 
by the characters above specified, more particularly by the amount of impurities 
they contain, the absence of fluid cavities, and by the minutely flecked or 
aggregated structure which they present between crossed Nicols. 
Arranged in the order of durability, the essential mineral constituents of 
these rocks would stand as follows :—The apatite remains singularly fresh, even 
in the midst of thoroughly kaolinized felspar. The quartz, where it occurs, is of 
course also unimpaired. Augite and titaniferous iron are about equally well 
preserved ; where the one shows signs of alteration the other is usually also 
affected. The felspars have been almost invariably attacked, every stage being 
observable from the pellucid crystal to the dull granular kaolin. The green and 
brown decomposition products may represent in part minerals no longer 
recognisable. 
