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
