Page:VCH Cornwall 1.djvu/73

 GEOLOGY their upper surfaces appear to be domes, it is by no means unlikely that they are huge laccolites, which have been forced laterally between the strata under somewhat similar conditions to the smaller injections which constitute the greenstone sills. Further, although derived from the same magma basin they may differ widely in geological age. Of these five larger bosses the Cam Menelez granite occupies the central position. As it is also the chief seat of the Cornish granite industry, as well as our most important mining centre, its composition and structure will be more particularly described, and the remaining granite masses will be touched upon later in so far as they depart from this type. In the Carn Menelez granite we shall include the two ad- jacent patches of Carnbrea and Carnmarth which are in such close con- tiguity as to lie within the same eruptive centre. This mass may be described as a grey and coarse textured granite consisting of orthoclase and plagioclase felspar, abundant quartz and both black and white mica, together with tourmaline in varying pro- portions. Lithia mica is a common constituent in parts of this granite. Porphyritic felspars are commonly distributed, and include individuals of idiomorphic orthoclase exhibiting twinning of the Carlsbad type. Andalusite and topaz are of rare occurrence. Although not so coarse as some of the other Cornish granites, which contain porphyritic crystals of large size, like those of the Land's End type or of the mass near St. Austell, yet the Carn Menelez rock presents a texture which, as compared with other British granites, is not only coarse as regards the matrix, but also in the size of its porphyritic felspars, which are often 2 or 3 inches in length. While both muscovite and biotite are usually present, every variation may be observed in their relative abundance. In some localities muscovite may occur almost to the exclusion of the biotite, while in others the mica may be almost entirely of the latter variety. Tourmaline is usually brown, sometimes blue and rarely green, and it occurs both as a constituent of the ground mass, in which it generally takes the form of stumpy prisms, and as a later introduction coating the faces of joint planes in needle-shaped crystals, which tend to dispose themselves in bunches, and sometimes attain a length of 2 or 3 inches. Tourmaline seems to be a conspicuous mineral in the more acid portions of the granite which vein the normal type. In some veins of coarse acid granite tourmaline occurs to the exclusion of biotite, and some of the individuals noted under these conditions are as large as half an inch in length. Where such coarse acid veins have been observed to contain tourmaline as well as massive schorl, contiguous veins which are finer in texture are found to be rich in biotite, with tourmaline entirely absent. In the coarse veins containing tourmaline the mica that occurs is muscovite, and other instances have been observed where the mica of the coarse acid veins is confined to the white variety. Although no sharp change of texture constituting a chilled margin has been noticed at the outer edges of this granite, yet if the mass be regarded on the large scale, the marginal portions are often finer grained than the interior, contain a larger proportion i 25 4