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. 



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