Vol. 4] 



Lawson. — The Robinson Mining District. 



327 



reason be regarded as representing a mass of peripheral blout 

 about the porphyry in the vicinity of the edge or margin of the 

 laccolith, which has since been caused to shrink in area by ero- 

 sion, leaving the more resistant blont behind as a residua!. If 

 this view be accepted for such occurrences, they would need no 

 special discussion as apart from the blont now found in the 

 porphyry areas. There are, however, occurrences which can not 

 be so explained. They lie wholly in the limestone and extend 

 down into it as irregularly bulging and tapering masses which 

 have some resemblances to veins but differ from these in the ex- 

 tremely abrupt and short lenticular habit which they affect. 



Varieties of Quartz Blout. — The quartz of the blouts is by 

 no means constant in character throughout the field. There are 

 a great many varieties. These need not, however, be described 

 in detail and it will suffice for purposes of description to class 

 them under four subdivisions made on structural grounds. These 

 are the (1) solid, compact quartz, (2) cavernous-weathering 

 quartz, (3) brecciated quartz, and (4) cellular quartz. The 

 first of these is the most common. It resembles closely a mas- 

 sive quartzite of granular but compact texture. Sometimes it 

 is nearly pure quartz, glassy or whitish or but slightly yellowish 

 in appearance, but for the most part it is deeply stained with 

 iron oxide. The colors are sometimes disposed in a eutaxitic 

 fashion, or it may be seamed with intersecting veinlets of more 

 deeply colored jaspery quartz. It weathers in smooth, rounded 

 knobs, or breaks down into a talus of irregularly angular blocks. 

 Occasionally this variety shows a stratiform appearance but with 

 no tendency to part along the planes between the different lay- 

 ers. The difference in the layers is usually due to both color 

 and texture, flinty or cherty layers alternating with those of a 

 more granular character. The cherty portions may be dark to 

 almost black or may be light colored — yellowish or milky white. 

 The granular portions, where not heavily stained by iron oxide, 

 may be glassy or gray or very dark. 



The cavernous variety of the blout quartz differs from the 

 last described by its weathering out in extremely irregular forms 

 with rugged chambers and straggling channels. In the bottom 



