Page:VCH Cornwall 1.djvu/74

 A HISTORY OF CORNWALL of biotite, frequently to the almost entire exclusion of white mica, and are heavier than the coarser grained material, so much so as to affect its commercial value. This increased specific gravity (about i per cent) is no doubt due to differentiation in the mass prior to consolidation by the concentration of the basic oxides to the cooler peripheral portions. Although this granite mass here and there shows veining of its evenly-textured material, by granite much finer in grain, more especially perhaps in the parish of Wendron, the evidence is clear that such flowage within the mass was effected before final consolidation, and does not represent two generations of granite. The finer veins frequently contain porphyritic felspars, identical both as regards size and form, with those of the adjoining normal type of granite, while in some instances the veins are so charged with these individuals that the fine-textured ground mass occupies but a subordinate position. Between Bolitho and Boswyn the fine textured granite covers a tract exceeding a square mile. The Land's End granite is far coarser than the Carn Menelez mass, and includes an area of about seven square miles of fine grained grey biotite granite with small scattered crystals of orthoclase and pinite, which according to Mr. Reid is of later age than the coarse granite into which it has been intruded. Much of the town of Penzance has been built of this stone. As the Carn Menelez granite is but rarely seen in actual contact with the killas the precise nature of its junction has not been observed, but judging from the margins of the other granite masses of which the coast affords so many examples, there is no reason to doubt that veins protrude from its mass into the adjoining killas. Beautiful examples of such veins are seen on the coast that fringes the Land's End granite mass, as in the vicinity of the Gurnard's Head, Cape Cornwall, Whitsand Cove, at Mousehole, at Trewavas near Porthleven and other localities and similar phenomena may be seen at St. Michael's Mount. These intrusions of molten rock have effected profound alteration on the killas into which they have been injected. The Carn Menelez mass presents an aureole of alteration which extends for about 1,000 yards from its margin, the innermost portions of which have been metamorphosed into mica schists crowded with andalusite, while the outer aureole consists of knotted slate. In the latter case mineralization is in the incipient stage, while in the former it is complete, the micas of the schists (both muscovite and biotite) being the products of crystalliza- tion, due to chemical action set up by the heated granite mass. The metamorphic aureole of the Land's End granite is still greater, while metamorphism has been more intense, the sedimentary rocks being indurated and hornfelsed to such an extent that it is often difficult to distinguish these rocks from the greenstones. The difference in area of the two masses is not great, but on the other hand they differ widely in texture, the Land's End mass being much coarser, and the explanation of its greater metamorphic action is to be looked for in its slower cooling. 26