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



341 



or a foot and a half in width, the black color being due to the 

 pulverization of the pyrite and chalcocite in the zone of crush- 

 ing. In other cases, there is a notable deposit of these sulphide 

 shears which has been made subsequent to the movement and 

 which, therefore, shows no evidence of crushing. In those cases 

 where the black gouge is present, it is most probable that the 

 movement was progressive, the first rupture having afforded 

 conditions favorable for the deposition of the sulphides, and the 

 recurrence of the movement having effected their attrition, to- 

 gether with the adjacent porphyry. In some cases there are 

 seams of fresh sulphides in the midst of the gouge. At the lower 

 limit of the zone of oxidation it is not uncommon to find that 

 the oxidation has extended downward along the slips and shears 

 below the general limits of the zone. It appears thus that these 

 structural features of the porphyry are important factors in the 

 process of mineralization, and it becomes an interesting question 

 to inquire into their origin. The contrast which is presented 

 between the intensely broken and sheared porphyry and the 

 unmoved condition of the surrounding country indicates at once 

 that the cause of the movement is an internal one, intrinsic in 

 the mass of the porphyry, and not either general or local earth 

 movements. The fact that the prevailing dip of the slip planes 

 and shear zones is in the same direction as the dip of the por- 

 phyry mass, though probably at steeper angles, indicates that 

 the movement has been one of repeated and complex normal 

 faulting, and that this faulting is but an expression of the ten- 

 dency of the mass to adjust itself to a new and diminished vol- 

 ume. It is further clear that such a tendency would not have 

 found expression in faulting, had the shrinkage of volume been 

 uniform in degree and in time throughout the mass. 



Our inferences thus lead us to a picture of the porphyry mass 

 shrinking in volume unequally in different parts and at different 

 times. The only cause that can be suggested for such a shrinkage 

 of volume is a chemical change within the porphyry and an elimi- 

 nation of certain of its chemical constituents. This suggestion 

 is well borne out by the abundant evidence of kaolinization and 

 allied changes within the body of the porphyry as far as it has 

 been yet explored. The change from orthoclase to kaolin, with 



