Vol. 4] Lawson. — The Robinson Mining District. 329 



anee here may well be due to other causes than the replacement 

 of an original sedimentary structure. 



There is still another variety of the blout quartz which merits 

 especial attention because its origin is not less clear than in the 

 case of the cavernous, stratiform varieties with residuals of 

 limestone. This is a light colored rock free from deep ocherous 

 stains, although it may be distinctly yellowish. It possesses a 

 prevailing spongy or cellular structure on weathered surfaces 

 and tends to break down into fragments of small size which 

 mantle the upper and flatter slopes of the porphyry areas. In 

 consequence of the tendency to break down, this variety of blout 

 does not present such bold croppings as in the case of the other 

 varieties. It is found occasionally, however, as on the Minne- 

 sota claim, as a cap of some prominence on the summits of the 

 hills. This variety of blout quartz occasionally shows traces of 

 porphyritic structure and it passes by insensible gradations, 

 representing stages of less intense silicification, into the well 

 characterized porphyry. 



It thus appears that the blouts, which form so prominent a 

 * feature of the porphyry areas, and which from their mapping, 

 appear to be remnants of a discontinuous shell or envelope which 

 once encased the porphyry, are in part the result of the replace- 

 ment of the adjacent limestone, and in part the result of the 

 extreme silicification of the porphyry itself on its margin, but 

 in greater part can not with confidence be referred to either of 

 these origins on the basis of direct evidence, although it seems 

 probable from analogy that most of it is due to the replacement 

 of limestone. 



Relations to Copper Ores and Garnet Rock. — The more or 

 less ocherous iron stain which affects most of the blout crop- 

 pings has generally attracted the attention of prospectors to 

 them and not a few pits have been sunk upon them. It is only 

 rarely, however, that ore is found in these croppings and it is 

 then, almost in every case, in the form of sparing quantities of 

 malachite, azurite and chrysocolla. A little fresh pyrite is not 

 infrequently found and it is the oxidation of this that gives the 

 blout its ocherous appearance. A number of shafts which have 

 been sunk in the blout appear to pass through it at no great 



