32 ECONOMIC GEOLOGY 



When the intruded rock is traversed by numerous joint planes, 

 the vapors decompose its various constituents and readily form 

 new minerals. If the country rock is not traversed by joints the 

 process begins along planes of bedding or cleavage planes of min- 

 erals and the alteration is easily effected. Feldspars are kao- 

 nized and micas chloritized. Silicification of the country rock 

 and greisenization also are not uncommon, but the character of 

 the change depends upon the nature of the magmatic vapors. 



Fluorine belongs to the earliest emanations but boron, carbon 

 dioxide, and hydrogen sulphide may belong either to the early 

 or later emanations. 



Order of Deposition. At Cornwall both the oxide of tin and the 

 sulphide ores were simultaneously deposited. The presence 

 of copper ores upon previously formed cassiterite leads to the 

 conclusion that copper ores continued to be deposited after the 

 deposition of tin had ceased. In other instances where tin and 

 copper ores arrived simultaneously, cassiterite was deposited 

 first. 



Thomas and MacAlister give the order of deposition: (1) Cas- 

 siterite and wolframite, together with the sulphides of copper, 

 iron and arsenic. (2) The sulphides of copper, iron and arsenic, 

 associated with those of lead, silver and cobalt. (3) Silver sul- 

 phides and the oxides and carbonate of iron. (4) Nickel, anti- 

 mony, and manganese minerals. 



The intrusion came in as large batholites of granite cutting 

 sedimentaries of Ordovician Age. At Central and East Corn- 

 wall they cut through Devonian rocks. The smaller batho- 

 lites at Devonshire cut Devonian and Carboniferous strata. 

 The ore is concentrated either in the periphery of the granite 

 batholith or the metamorphic aureole. 



The copper ores, containing chalcopyrite, chalcdcite and 

 bornite, where tin does not occur, are often of pneumatolytic 

 origin. In Norway where copper lodes are associated with a 

 greisenized granite, the ore is unquestionably formed under 

 the principles of pneumatolysis. 



At Copperfield, Strafford. and Corinth, Vermont, the chal- 

 copyrite, associated with tourmaline, occurs in saddle-shaped 

 bodies in mica schist, or in chimneys at the contact of granite 

 veins with mica schist. Pyrite and pyrrhotite are the associated 

 sulphides. These sulphides were formed under pneumatolytic 

 conditions. 



