]8 Transactions. 



magmas. The emanations, he contends, must have prepared 

 the way by introducing into the enclosing rocks, or simply by 

 depositing in the vein fissures, elements such as sulphides, 

 fluorides, chlorides, &c, which, subsequently dissolved anew 

 by the circulation of superficial waters, have rendered the 

 latter essential aid in the processes of alteration.* 



The extent of contact metamorphism effected by the 

 granite intrusions of Albany, in New Hampshire, was fully 

 investigated by Hawes.f His analyses showed a progressive 

 series of changes in the schists as they approached the 

 granite. The rocks are dehydrated, boric and silicic acids 

 have been added to them, and there appears to have been an 

 infusion of alkali on the line of contact. He regarded the 

 schists as having been impregnated by hot vapours and solu- 

 tions emanating from the granite. 



Contact deposits frequently lie at the boundary between 

 the eruptive and the country rock ; also at variable distances 

 from the eruptive, but never outside the zone of metamor- 

 phism. More particularly, contact ores occur in limestones, 

 marly and clay slates, and are accompanied by the usual 

 contact minerals, garnet, vesuvianite, scapolite, wollastonite, 

 augite, mica, hornblende, &c, and in clay-slate by chiasto- 

 lite, &c. Contact ores are principally magnetite and specular 

 iron, but sulphides of copper, lead, and zinc often occur. 

 Pyritic contact deposits are typically represented by those of 

 Vegsnas, in Norway ; Eio Tinto, Tharsis, and San Domingo, 

 in Spain. 



The pyritic ore-mass in Mount Lyell Mine, in Tasmania, 

 is generally described as a contact deposit, although its 

 geologic occurrence does not strictly conform to the common 

 definition of such a body. It is a boat-shaped body lying 

 between talcose schists and conglomerates.! The mine-work- 

 ings have shown that it gradually tapers downwards from the 

 outcrop, being cut off below by a great thrust-plane. There 

 are no eruptives in actual contact with the ore-body, but 

 dykes of diabase and other igneous rocks occur in the district 

 at no great distance. The existence of these dykes and of 

 bands of schist impregnated with sulphides forming fahikands 

 would lead to the belief that there at one time existed chan- 

 nels of communication leading from the eruptive rocks to the 

 vein cavities. It seems probable that the ore-bodies in the 

 Mount Lyell field were formed in the later or solfataric stages 

 of eruptive after-actions. 



* L. de Launay, "The Genesis of Ore-deposits," 1901, Discussion, 

 p. 610. 



fG. W. Hawes, Amer. Jour. Sci., vol. xxi, 1881, p. 21. 



I Prof. J. W. Gregory, "The Mount Lyell Mining Field," Trans. 

 Aust. Inst. Min. Eng., vol. i, part iv, July 1904, p. 281. 



