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



325 



outs." This discrimination appears to the writer to he well 

 warranted. They cannot be regarded as veins or lodes; and 

 while, as will be shown later, they are to a large extent replace- 

 ments of other rocks, they may not be wholly so. They consti- 

 tute a particular mode of occurrence of quartz which the writer 

 desires to distinguish both from vein quartz and from replace- 

 ments of country rock by silica in the vicinity of veins. To 

 signalize the distinction and to simplify their discussion, the 

 miners' term "blow out" will be adopted in the modified form 

 of blout, and. they will, therefore, be referred to henceforth as 

 quartz blout, much in the same sense as one would use the ex- 

 pression "quai'tz vein." Similarly as we designate the quartz 

 of a vein specifically as "vein quartz," so here we may refer to 

 the quartz of which the blouts are composed as blout quartz. 



Field Relations. — The most striking feature of these quartz 

 blouts is their vast size as compared with ordinary veins. A 

 glance at the map will show that in the vicinity of the Ruth 

 mine they occupy a very considerable fraction of the area of 

 the porphyry ; perhaps one-third is occupied by the blout. 



The mapping indicates that the greater part of the quartz 

 blout lies on the periphery of the porphyry mass and occupies 

 an intermediate position between it and the surrounding lime- 

 stone ; and although some important occurrences are more cen- 

 trally situated, these from the mapping appear to be of the 

 nature of caps upon the hill tops and ridges. If we accept the 

 suggestion thrown out in an earlier portion of this paper, that 

 the porphyry mass is of the nature of a laccolith, then these cen- 

 trally situated quartz blouts are also peripheral, and represent 

 a contact development between the porphyry and its roof of 

 limestone now removed by erosion. Indeed, we can form a very 

 consistent hypothesis to explain the distribution of the blouts 

 by supposing that they were once a more or less continuous 

 envelope, incasing the porphyry, particularly on its upper side, 

 and intervening between it and the over arched roof of lime- 

 stone. The removal of the limestone roof by erosion and the 

 dissection of the envelope of quartz blout, leaving exceptionally 

 thick or resistant portions of it as residuals reposing upon the 

 porphyry, would well explain most of the occurrences. This 



