486 F. W. Clarke — Some Nickel Ores from Oregon. 



with a faint suggestion of fine granular structure. Generally 

 it is dull, but where most compact and traversed by a series of 

 minute fissures or seams of quartz it has a decidedly waxy 

 luster. Under the microscope it usually appears to be an 

 aggregation of irregular grains which have in transmitted light 

 a pale yellowish green to coffee-brown color, and a peculiarly 

 clouded waxy aspect. Where the grains are very thin the 

 genthite may be said to be transparent and isotropic, but the 

 majority of them are only translucent. In the narrow seam 

 of genthite lying between seams of quartz the former is indis- 

 tinctly fibrous and feebly double-refracting ; but its system of 

 1. crystallization could not be defi- 



nitely determined. In small 

 veins it is free from grains of 

 other minerals, but elsewhere it 

 is very intimately commingled 

 with quartz. The relation of 

 the two minerals is shown in 

 the accompanying figure 1, in 

 which the shaded portions (3) 

 are genthite, and the clear 

 one (4) are quartz. The com- 

 mingling of the two minerals is 

 so intimate as to make it evident 

 that both were deposited from 

 solution in circulating waters. Yeinlets of quartz are fre- 

 quently found cutting across those of genthite, and in general 

 it appears to be true that the latter mineral was laid down first, 

 although it is probable that both were precipitated at about the 

 same time. Although the purest genthite is to be found with 

 quartz, the mineral is more commonly associated with serpen- 

 tine ; and this relation is a most 

 important one in its genetic 

 significance. The accompany- 

 ing figure 2 represents the edge 

 of one of the larger veins of gen- 

 thite (3), with numerous vein- 

 lets or fissures extending out 

 into the serpentine (2). The 

 tributaries are abundant on both sides of the vein. Figure 3 

 shows a vein of genthite (3) in the serpentine (2), which en- 

 velopes small masses of residuary oxide of iron (1), left by 

 the decomposing olivine. The area represented is only 0*64 

 of a millimeter in diameter and contains no olivine ; but less 

 than half a millimeter .from the boundary of the vein much 

 olivine still remains, although deeply coated with oxide of 

 iron and serpentine. The branching streamlets from the vein 



