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 whereas the augite has been replaced by a mat of actinolite (horn- 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 



