PRECIOUS METALS 83 



Porcupine. The porcupine district represents a new field. It 

 is situated in the northern part of Ontario. The most important 

 counties thus far exploited are Doloro, Shaw, Tisdale and 

 Whitney. In these four counties practically all of the ground 

 has been staked. The Dome mines represent the pioneer work 

 in this field. Within the first 100 ft. from the grass roots the 

 company is said to have actually blocked out $8,000,000 of gold. 

 The field bids fair to be a large producer for three reasons. (1) 

 Its own native richness; (2) the great number of scattered free 

 gold discoveries, and (3) the completion of the railroad to Por- 

 cupine during the summer of 1911. The geology of the district 

 is represented by a series of pre-Cambrian metamorphics traversed 



FIG. 62. Structural arrangement of the Silurian slates and sandstones 

 at Bendigo, Australia, in which the auriferous saddle-reefs are found. 

 (After Thomas and MacAlister's Geology of Ore Deposits.) 



by a diabase of post-middle Huronian age. The gold is free mil- 

 ling and the most important gangue is quartz. (See Fig. 61.) 

 The Geological Horizon of Gold : Gold may be found in small 

 quantities in nearly all, if not all, geological formations. It is 

 especially abundant in the pre-Cambrian, Ordovician, Cretaceous 

 and Tertiary formations, that is, in general, in the older rocks, but 

 the last two are among the younger formations. The Silurian 

 Devonian and Carboniferous terranes are not known to carry 

 gold in paying quantities in the United States, but in British 

 Columbia gold occurs in the Carboniferous strata. However it 

 is possible that some of the gold-bearing rocks of the Appalachian 

 belt are as late as the Carboniferous, but in the main they are 

 Cambrian and Ordovician. (See Fig. 62.) 



