12 R. Arnold — Rocks from the Sawtooth Range. 



the chert in one wall of the vein on the east flank of Mt. 

 Henderson. The surface of this rock weathers into minute 

 pits so characteristic of certain limestones. The red blotches 

 in the rock are said to sometimes carry small particles of native 

 copper. 



A specimen of material said to occur as float in the region 

 of Copper Mountain, 6 miles south of Mt. Skokomish, consists 

 of mineralized red jasper and gray quartz. 



Igneous Rocks. 



The igneous rocks in the collection embrace typical diabase, 

 a fine-grained diabase, and a peculiar fibrous serpentine resem- 

 bling antigorite. The igneous rocks are all younger than the 

 sedimentaries, occurring as dikes intruding the latter, usually 

 with a north to northeast trend, parallel with the strike of the 

 sedimentaries. 



The most typical example of diabase occurs as a large dike 

 at Smith's Camp, and lies adjacent to a dark slate spotted with 

 aggregations of white quartz. The diabase is moderately fine- 

 grained, greenish to greenish gray in color, and breaks with a 

 rough irregular surface. Thin veins of chlorite occur in some 

 of the joint cracks. Under the microscope, the rock shows 

 typical ophitic texture. Plagioclase and augite are the most 

 important minerals, the former predominating. The plagio- 

 clase occurs in lath-shaped crystals ; the augite is slightly pleo- 

 chroic, and is altered in many cases. Titaniferous magnetite 

 is found abundantly in isolated grains. Calcite is one of the 

 alteration products. This diabase is a rock that could properly 

 be called a greenstone. 



Diabase also occurs abundantly, intruding the slates, on the 

 west side of Box Canyon, where it has been prospected con- 

 siderably, but with negative results. This rock is fine-grained, 

 light greenish to greenish gray and breaks with a rough frac- 

 ture. Pyrites are plainly visible in small but numerous specks 

 throughout it. In thin sections, it is seen to be less typically 

 ophitic in texture than the diabase last described. It contains 

 about equal quantities of plagioclase and augite, the latter more 

 pleochroic than in the diabase previously mentioned ; quantities 

 of titaniferous magnetite and iron pyrites also occur through- 

 out the mass. Chlorite appears to be the principal product of 

 alteration. 



A specimen of a fine-grained diabase, approaching a basalt in 

 appearance, occurs as one of the igneous rocks at Camp Black 

 and White. In hand specimens, it is fine-grained, very dark 

 colored and breaks with knife-like edges along several irregular 

 systems of joint planes, parallel with which are sometimes thin 



