A HISTORY OF CORNWALL 



discussion of the long array of mineral substances which have been pro- 

 duced in the granites and the neighbouring killas by chemical action since 

 the consolidation of the granite, many of which are of economic value 

 and constitute the metalliferous ores, would be beyond the scope of this 

 article. Mr. J. H. Collins has compiled a list of eighty-six minerals that 

 occur in the Hensbarrow granite and in the killas aureole in close 

 proximity. 



The granite of the Land's End district is characterized by a general 

 uniformity of texture, notwithstanding* the finer grained portions to 

 which attention has been drawn, by the coarseness of that texture 

 exhibited in the large size of its porphyritic felspars, as well as by 

 the frequent presence of pinite. Schorlaceous granite is a common 

 feature along its northern margin, beautiful varieties being seen in the 

 vicinity of St. Ives, where the larger porphyritic felspars are replaced 

 by schorl and quartz, in which the original outlines of the felspars 

 are still preserved. In these pseudomorphs the preservation of the casts 

 of the earlier mineral by retaining its crystalline form, leads to its precise 

 identification notwithstanding the complete removal of its substance. 



The granite of the Scilly Islands is usually a somewhat coarser 

 variety in which finer material appears in the form of veins, while 

 schorl is apparently rare. 



The granite of Godolphin Hill is variable in type, and according 

 to Dr. Boase is in part extremely rich in felspar, which acts as a base in 

 which white crystals of felspar, nodules of clear quartz and black mica 

 are thickly embedded, while schorlaceous veins are common in it. 



The granite of St. Michael's Mount is generally fine grained, with 

 a small proportion of mica. The joint planes of this granite often con- 

 tain quartz, topaz, cassiterite and other minerals, and are bounded by 

 bands of greisen which pass imperceptibly into the granite. 



The very slow consolidation of the granite masses has resulted in 

 their complete crystallization. The constituent crystals however have 

 mutually interfered with one another so that they interlock. In 

 the earlier generations of the felspar represented by the porphyritic forms, 

 we frequently see perfect crystals, while mica and tourmaline likewise 

 occur idiomorphic ; the smaller felspars on the other hand have a less 

 perfect contour, while the quartz which in normal granite has been the 

 last constituent to consolidate has been forced to accommodate itself to 

 the interspaces which remain. When however cavities occur in the rock 

 we find all the various minerals lining their walls in well formed crystals, 

 which are frequently of large size and beauty, especially the purer 

 varieties of silica such as rock crystal, amethyst, etc., while in such 

 situations topaz and other minerals which fall under the category of gems 

 have also been obtained. 



Such cavities probably owe their origin to the imprisonment of 

 steam, and in this connection it may be pointed out that the quartz itself 

 which enters into the composition of the granite, and the crystallization 

 of which mineral marks the final consolidation of the rock, contains 



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