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 28
