/02 THE TIN-DEPOSITS OF NEW ENGLAND, N.S W , I., 



has been carried on mainly near the margin of the granite where 

 that rock is in contact with altered slate or greenstone. The 

 distribution of the ores is, however, extremely partial, for the 

 .margin for long distances yields nothing of value, whilst other 

 parts are exceptionally rich. So irregular is the distribution of 

 the lodes, as laid down on the map, that at first sight it does not 

 seem necessarily to indicate any close connection with the 

 granitic intrusions, for many of the lodes are far distant from 

 any granite mass seen at the surface. When, however, we take 

 into account the contour of the buried granite masses and their 

 proximity to the surface, as suggested by the extent of the meta- 

 morphism, and also the probable nearness in certain places of the 

 slaty dome which once covered what is now bare granite, the 

 close relation of the lodes to the intrusion of the granite becomes 

 more evident." 



The deposits at Altenberg and at Zinnwald exhibit the same 

 characteristic distribution. Here the tin-bearing rock is a 

 granite intrusiv^e into granite-porphyry at Altenberg, and inta 

 *' Teplitz " quartz-porphyry at Zinnwald. The tin occurs in both 

 places as stockwork, the impregnated granite receiving the name 

 of " Zwitter " rock. The granite itself, as far as can be judged 

 by handspecimens, is very similar to a fine-grained tin-bearing 

 granite containing microcline, to the south of Tingha. 



Whilst it is true that most of the tin-deposits in this part of 

 New England occur at or near the margin of the intrusive " Acid 

 Granite," the converse, that all such contacts are tin-bearing^ 

 cannot be said to be true. As in Cornwall much, if not most, of 

 the contact-zone is barren or unproductive. 



The System of Fractures. 



For a genetic investigation of ore-deposits, few factors are 

 more important than the relations of the ore-bodies to the systems 

 of fracture of the country. The importance of these relations 

 was early recognised by Elie de Beaumont, in his work on the 

 Fracture-Systems of Europe. An investigation of the fissure- 



