40 



as well as horizontally ; the minute, and very laborious investiga- 

 tion of which is the foundation of the present observations. The 

 crystalline series in Europe falls into insignificance when com- 

 pared with those of America, and it is in such extensive areas 

 that the real character of the crystalline base can be clearly as- 

 certained. 



Besides the regular transition of the crystalline base into slate, 

 there are also veins formed, interlaminated in the mass. Should 

 the base contain a large proportion of magnesia and talc, veins 

 of serpentine rocks will be formed ; should the granite predo- 

 minate in silica, quartz veins will be found very abundant. 



The felspathic granite never produces slate for the want of 

 mica, therefore it is generally covered by a massive rock which 

 is erroneously called clay-slate. Should hornblende and lime be 

 disseminated 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, wdth variable degrees of intensity. The porphyritic 

 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 di- 

 stinct 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 Cornwall have a porphyritic gra- 

 nite, which on the surface is partially decomposed, forming 

 patches of dark masses of the same substance ; the intermediate 

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

 porpyhry. 



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 





