Geology and Magnetism. 517 



The felspathic granite never produces slate for the want of mica, 

 therefore it is generally covered by a massive rock which is erro- 

 neously called clay-slate. Should hornblende and lime be dissemi- 

 nated in a felspathic base, hornblende, basaltic, trap, and greenstone 

 veins are formed ; and as carbonic acid is generally combined with 

 the ingredients, this compound base produces great disturbances in 

 the superincumbent masses ; it is the most restless base of the 

 whole of the above compounds. However, we must always bear in 

 mind that there is no granitic base in situ actually dormant ; these 

 are constantly acted upon by the polar current, with variable 

 degrees of intensity. The porphyritie granite is the richest base 

 for producing minerals in Chili, Peru, Quito, New Granada, 

 Mexico, England and Germany. The quartzose granite is the most 

 unproductive. 



When the granite forms a moist massive base, it is seldom de- 

 ficient in mineral salts, but when comparatively dry, with a distinct 

 crystalline grain, it is generally poor in mineral. These are the 

 primary points to be first considered in making a survey of a mineral 

 district. 



The metalliferous parts of Crownwall have a porphyritie gra- 

 nite, which on the surface is partilly decomposed, forming patches 

 of dark masses of the same substance ; the intermediate part 

 (forming the transition) is called " elvan," being a fine grain 

 porphyry. 



The transition of the granite into the slate is very irregular, 

 owing to the excess of felspar and the want of mica : this is also 

 the cause of the series not possessing the uniform cleavage struc- 

 ture seen in America and other places. However, there are a 

 few meridional channels intersecting Cornwall, presenting the usual 

 phenomena of the vertical polar cleavage, and cleaving the superin- 

 cumbent beds, which may be seen at the United Hills, St. Agnes, 

 and various other parts of the northern coast. The interlamination 

 of the granite, porphyry, and slate, and the common northward 

 transition, may be seen at Dolcoath, and along the whole range 

 of the Cam Brea granite, on the north side. 



The gradation of granite into gneiss, and gneiss into mica and 

 clay-slates, may be seen in Wicklow, and also in the western part 



3 x 



