496 PROCEEDINGS OF THE NEW YORK MEETING 



prospectors, unless their attention had l)een previously called to their character 

 and importance. 



Even when veins of this nature contain hut little gold at the surface they are 

 generally found to be richer in depth, and from their supposed mode of formation 

 the author attempts to give a rational explanation of this circumstance. 



They are evidently true Assures, as indicated by the faulting which may be seen 

 along some of them, and also from the fact that they intersect alike both the mass- 

 ive and the stratified rocks. The mikadoite spreads out irregularly on either side 

 of the quartz sheet which accompanies it and forms, as it were, the backbone of 

 the vein. The latter may carry small quantities of the sulphides of copper, lead, 

 zinc, and more rarely bismuth, but these minerals are scarcer in the mikadoite, in 

 which the fine grained pyrite above mentioned is the prevailing sulphide. 



Many of the felsite and greenstone dikes of Bog Bay district have been found by 

 assay to be auriferous and a few of them are being worked for gold with promis- 

 ing results. ' 



It seems highly probable to the author that the gold was derived from heated 

 vapors and solutions which ascended through the fissures from great depths, pre- 

 sumalily from the vicinity of the magmas — the source of the felsites and green- 

 stones. 



The felsite dikes are generally present near the veins and are usually more or 

 less charged with the fine grained sulphide of iron that almost invariably accom- 

 panies the gold in the veins. Their auriferous character is more marked when 

 they are close to tlie quartz veins. The felsite in many places loses the crystalline 

 texture and passes into phonolite with the usual metallic ring. It is probable that 

 this rock may liave some connection with the occurrence of the gold here, as the 

 phonolites of the famous Cripple Creek district of Colorado are admitted to have 

 in that region. In a somewhat similar way the trappean eruptions of the Kewee- 

 naw district are believed to be connected with the presence of native copper 

 there. For these reasons the auriferous veins under consideration may be ex- 

 pected to continue productive to a considerable depth, and to improve rather than 

 the reverse at lower depths. 



In regard to the manner in which tliese veins were formed, the author, after 

 much study and observation, reaches tlie conclusion that during the movements 

 wliich caused these rents a sufficient pressure was exerted to i^revent a separation 

 of the walls so as to leave openings for the reception of the vein matter ; hence the 

 openings now filled with gangue must have been subsequently created. Tlie 

 heated solutions that would be certain to percolate through these fissures Avould 

 dissolve the silica out of the wall-rocks and deposit it as vein-quartz, while the 

 crushed rock adjoining the fissures would be metamorphosed in the manner de- 

 scribed by Professor C. R. Van Hise in his admirable paper " Metamorphism of 

 Rocks and Rock-fio wage." Referring to the same subject, Dr M. E. Wadsworth, 

 states in his pamphlet "The Lake Superior Copper Deposits," issued in 1891, that 

 " one of the latest phases of the formation of deposits of value has been the filling 

 in of fissures by the water-deposited quartz and other vein materials, or, in case no 

 crack nor fissure existed, by the removal of the country rocks along certain lines 

 and their replacement by vein matter." 



Attention is called to the great difference in character between the gold-bearing 

 fissure veins of Archean time and those of later age, such as the silver-bearing veins 

 of the Thunder Bay district. 



