108 OVERLYING AND TRAP ROCKS. containing no iron. All the grey varieties owe their colours to the protoxyde ; as intensity of colour is a sufficient test of its proportions. From the peroxyde are derived the varieties of red and brown ; and it is worthy of remark that the porphyritic veins found in granite, are generally characterized by these colours, while grey or black predominates in those that occur in the secondary strata. The term Basalt has been hitherto applied to every trap rock characterized by uniformity and minuteness of texture, united to a black colour ; but these do not form a mineralogical species. Thus it has included the dark and hard claystoncs and clinkstones, and the greenstones of a minute intermixture; while, as 1 have ascertained, the line augitic greenstones, together with pure augit in a minute condensed crystallization, have equally been confounded under this term, by the multitude, more ready with names than acquainted with minerals or rocks. Let it at least be a definite rock ; and as all the others are classed, it should lie limited to a minute aggregate of hornblende alone. Greenstone has been supposed to include the dark coloured mixtures of hornblende and compact felspar; but I have just said that colour is not a true ground of distinction between them and Syenite. But the very term itself is the produce of that mineralogical ignorance which I have tried to correct by the esta blishment of Augit rock. This is often a greenish substance, but the mixtures of felspar and hornblende never are so ; the reputed mineralogist who has mis led all that ever believed in him, appears to have con founded augit and hornblende. Greenstone must now mean the mixtures of hornblende, yet not with compact felspar alone, but with clinkstone and com mon felspar also, equally overlooked ; while it is