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air, in consequence of the decomposition of iron pyrites. This mineral not only coats the joints of this rock, but is also some times disseminated throughout its substance in small amyg- daloidal granules. Each of the preceding schistose rocks, but more particu larly the greenstone, gradually passes into thick lamellar slates, which exhibit various shades of blue and green, are soft, and abound in metalliferous veins, particularly of copper, the matrix of which is composed of quartz and dark green crystalline chlorite: they also contain layers or beds, which are compact and earthy, differing only from the slate in wanting the fissile structure. Thus we find, that the rocks of the schistose group imme diately surrounding the granite of Cornwall, may be referred to five distinct genera, viz. cornubianite, proteolite, green stone, actynolite-rock, and chlorite-rock. To these it is pro posed to subjoin a description of the magnesian rocks, the true position of which has perhaps not been decidedly ascer tained, though it appears in Cornwall to be in the lowest part of the calcareous group, the upper part of which contains organic remains, and therefore it may possibly belong to the intermediate or transition class. The magnesian rocks of this county may be divided into three genera: diallage-rock or euphotide, serpentine, and talc-schist; the last is generally received as a distinct rock, and, indeed, in other countries, it is extended over large tracts, with different associations; otherwise it might in Cornwall be regarded merely as a schistose species of ser pentine. The euphotide consists of felspar and diallage, both of which are very crystalline, and are generally very distinct, in the form of various sized crystals aggregated together, and mutually penetrating each other, after the manner of granite: its felspar does not appear to be identical with that of granite, which may be owing to the presence of magnesia, just as