300 



IRVING. 



shoots occur prevailingly on the basal Cambrian quartzite is 

 probably due to the position of this easily replaceable limestone 

 rather than to any influence exerted by the quartzite itself. 



Fig. i6. Siliceous ore vertical occurring along the contact of a quartz -porphyry 

 dike near the opening of the Big Bonanza Mine. 



TJie Big Bonanza. — The Big Bonanza Mine is situated on the 

 southern bank of Fantail Gulch, just at the lower end of the 

 town of Terry. The basal quartzite is here exposed high up on 

 the side of the gulch and the ore has been opened up by a 

 drift. The shoot is of an irregular character, and occupies 

 nearly the entire area covered by the claim. It attains in places 

 a thickness of fifteen feet, and is of both the blue or unoxidized, 

 and the red or oxidized varieties. The floor is of the usual 

 quartzite, but the roof is sometimes porphyry, and at others 

 shales. Verticals occur as usual, with a prevailing north and 

 south trend. An interesting vertical on the contact of a quartz- 

 porphyry dike was seen (see Fig. i6) near the mouth of the 

 main drift. 



Of especially frequent occurrence in this mine is the " barren 

 sand rock," by which is meant the hard blue crystalline lime- 

 stone mentioned above. At one point the ore shoot is com- 

 pletely cut off by this rock, but it is seen again at some distance. 

 The limestone forms a thick mass like a dike in close contact 



