662 REVIEWS 



(b) those in the Cambrian; (c) those in the Carboniferous; (d) those in 

 eruptive rocks; and (e) those in rocks of recent formation. 



The ore deposits in the Algonkian rocks include gold ores, copper ores, 

 and tin ores. Of these the gold ores are the most important. The gold 

 ores are also the chief ores in the region as a whole. The celebrated Home- 

 stake mines are working these ores. 



However frequent the erviptive dikes and sills in the various forma- 

 tions, it cannot be said that the origin of the ores is due directly to the 

 presence of igneous rocks, though they may be the ultimate source of the 

 mineralizing solutions which made the deposits. 



The ores in the Algonkian rocks occupy fractures and crushed portions 

 in the schists, and are generally found to be richest beneath the overlying 

 impervious shales of the Cambrian system. There seem to have been two 

 principal periods of mineralization, one previous to the rhyolitic intrusions, 

 and the other later. 



The copper ores occur in small patches in the schists, and, so far as 

 known, associated with graphitic schists, the graphite being supposed to 

 have reduced the copper from the copper solutions. 



The tin, in the form of cassiterite, occurs in two forms: (i) in peg- 

 matitic granite, and (2) in placers. These deposits were never very exten- 

 sively worked, and have been almost entirely given up since the collapse 

 of the tin enterprise in the southern part of the hills. The tin is too much 

 scattered thoughout the rocks ever to make it worth while to carry on 

 extensive mining. 



The ores in the Cambrian rocks are the ores of gold and silver in three 

 forms, of which the second is the most important. These are: (i) gold- 

 bearing conglomerates (fossil placers); (2) the refractory silicious ores; and 

 (3) pyritous ores. Besides the gold and silver ores there are ores of tung- 

 sten, and lead and silver. The second group of gold and silver ores are 

 called refractory because amalgamation has so far failed in their treatment, 

 there being a great deal of secondary silica, pyrite, and fluorite in their 

 constitution. They occur in channel-like bodies usually along the bedding 

 planes of the dolomite, sometimes just beneath an impervious shale, and 

 sometimes just below sills of igneous rock. 



The ores in the Carboniferous limestone consist of two classes, neither 

 of which is very important. They are likewise refractory, silicious ores of 

 gold and silver, and ores of lead and silver. They were deposited, it is 

 presumed, by ascending waters in fractures in the massive limestone, but 

 without any particular concentration due to impervious beds, as in the 

 earlier formations. 



