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 

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