GEOLOGY 



or may occur as perfectly rectangular fragments, so that some mica-traps 

 have a brecciated appearance. The rocks thus vary considerably both 

 in texture and composition, and they appear to have been intruded after 

 the slates had been cleft but before the final cessations of the stresses, 

 some of the joints of the sills being parallel to the movement planes in 

 the adjoining slates, while there is a tendency to a parallel structure corre- 

 sponding with these joints. Moreover the minor dislocations in the 

 slates have heaved the mica traps. The mechanical deformation they 

 have suffered is slight, and is limited to the distortion of the mica, as is 

 rendered evident when they are examined under the microscope. The 

 dominant felspar of these rocks is orthoclase, and augite has been 

 recognized, while a striking characteristic of some of the dykes is the 

 abundance of apatite they contain. They are conspicuously developed 

 in the Fal estuary between Malpas and Mesack Point. 



Besides the igneous intrusions represented by granite, elvan and 

 mica trap, the killas has been invaded by another class of volcanic rock 

 more ancient than the foregoing, the products of which are known as 

 Greenstone. 



These rocks, although not so widely distributed as the elvans, are 

 not so restricted as the mica traps. Like the former they occur in 

 greatest abundance in the neighbourhood of granite masses, as is 

 particularly emphasized in the western division of the county by the 

 greenstone masses which partly encircle the Land's End and Carn 

 Menelez granites. The intrusive greenstones occur in the form of 

 elongated sill -like masses, and represent the heavier material of the 

 magmatic reservoir. Not only do they contain less felspar than the 

 acid intrusives already described, but that mineral is more commonly 

 plagioclase, the soda variety ; quartz has almost disappeared, while the 

 ferro-magnesian constituents are strongly represented. 



Hornblende is undoubtedly dominant in the rocks, and the 

 prevailing green colour of this mineral and of its decomposition products 

 has given rise to the name of Greenstone. As these rocks have been 

 involved in the earth movements which have deformed the slates they 

 exhibit various stages of the process of deformation. Moreover their 

 great development within the granite aureoles of contact alteration has 

 involved thermal metamorphism in addition. As the result of these 

 changes the rocks have often been profoundly modified from their original 

 conditions not only as regards structure, but likewise as regards mineral 

 composition. 



The researches of Messrs. Allport and Phillips conclusively demon- 

 strate that these intrusions were originally dolerites and basalts in which 

 the augite has been almost entirely replaced by hornblende. While 

 the more massive varieties of the rock which have resisted dynamic 

 metamorphism still retain some of their original felspar, yet the bulk 

 of that mineral is represented by a later generation, 

 i 33 5 



