Page:VCH Cornwall 1.djvu/82

 A HISTORY OF CORNWALL The greenstones which flank the Cam Menelez granite vary from coarse amphibolites or epidiorites to fine grained rocks of basaltic texture, and from masses which are highly sheared to others which have suffered no mechanical deformation. Microscopic examination leaves no doubt that the coarser masses are altered gabbros, while those of finer texture were for the most part originally basalts. At Bellevue near Penryn the massive amphibolite still retains its original felspar, whereas the augite has been replaced by a mat of actinolite (horn- blende), and iron ore is in process of alteration to granular sphene. Near Ponsanooth the rock shows a little foliation, the felspar which is less abundant is mostly in a granulitized condition, and besides some magne- tite the rock contains pale brown flakes of mica, probably the direct re- sult of the contact alteration by the granite. The sill between Pengreep and Treviskey is more variable in texture ; while portions are as coarse as that just described, the greater part of the rock is of fine grain. Moreover, as at Devis, the mass has undergone considerable mechanical movement, the finer, more compact zones and those of coarser crystal- line condition having been so packed and squeezed together that the coarser portions exhibit shearing. This rock strongly approaches the type of greenstone seen near St. Ives, which has been so profoundly metamorphosed by the granite of that region, and some of its structures recall the banded hornblendic rocks of the Lizard area. These greenstones may be taken as typical of this class of basic intrusion of Cornwall. While their principal constituents are felspar and hornblende considerable variation is seen in the relative proportions of those constituents. Moreover in the crushed varieties the secondary products may be so widely diffused that the original character of the rock is entirely destroyed. The intrusive greenstones however are not confined to the basic phase, but rocks of intermediate composition find their place within that group. Both from Newlyn and Flushing greenstones have been determined both by microscopic examination and chemical analysis to belong to the Andesite group. Although these rocks are much altered they have suffered little or no mechanical deformation. The Newlyn rock is unsheared and but little modified ; the felspar is fairly fresh, and the alteration of augite to hornblende is mainly paramorphic. The Flushing rock occurs outside the aureole of granite metamorphism and is more decomposed, the hornblende and biotite being represented by chloritic and serpentinous material. The varieties which have been metamorphosed into serpentinous products, such as the olivine dolerite of Clicker Tor and the serpentinous greenstone of Duporth, in which also olivine was probably an original constituent, represent an extreme phase of the basic greenstones. Some greenstones occur at Camborne extensively veined by massive garnet, some- times associated with epidote and pyroxene, all of these minerals having taken their present crystalline form subsequent to the shearing of the rock. Gametiferous greenstones have also been noted in other parts of Cornwall. 34