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DESCRIPTION OF TIIE PRIMARY [Ch. IV. 4-6 diallage differs from hornblende in containing a large pro portion of this earth. The constituents of euphotide, in some places, gradually become so small, that the rock is at length perfectly homo geneous ; and where this is interlaminated with small scales of shining diallage, the rock acquires a schistose structure. This schistose euphotide has been commonly called hornblende slate; but it is so different from all the varieties of the latter, and has such a characteristic appearance, that it ought to be distinguished. It appears, sometimes, to be imbedded both in the euphotide and in the serpentine; and the latter rock, even in some instances, seems to be subordinate to the schist. The fact is, that the schist often forms the connecting link by which euphotide and serpentine graduate into each other; and though these rocks alternate on a given line, like the massive and schistose greenstones, yet they do not preserve a continuous course for any considerable distance ; but mutually abut against, or are dovetailed into, each other; sometimes the one, and sometimes the other prevailing. The serpentine of Cornwall is proved to be a compound of diallage and felspar; or perhaps, rather, of compact felspar, by its frequent transitions into euphotide. This rock ex hibits a great many varieties; some of which are hard, whilst others are so soft as to yield to the nail. This difference ap pears to depend on the felspar base, which undergoes several modifications, between a crystalline, compact, and granular state, as seen in the precious, common, steatitic, and ollareous serpentines; in the same manner as the rocks of the porphy- ritic group assume various aspects, according to the composi tion of the compact felspar base ; with this difference, however, that in these, the proportion of the silica modifies the com pound, whereas, in the serpentine, the changes are attributable to the relative quantity of magnesia. The accessary mineral, diallage, also, imparts characters to the serpentine, according as it is intimately combined with the base, or is disposed in distinct forms. And, lastly, both of the component minerals,