LINDOREX.] ROCKS OF MOORE CREEK VALLEY. 6G9 



rapid accumulation in a body of water affording no opportunity for 

 concentration. Moreover, the detritus was mostly derived from the 

 immediately surrounding hills, which are nearly barren of mineral 

 veins, while the overlying gravels were transported, concentrated, 

 and assorted by streams coming from the region of auriferous quartz 

 veins. It is probable that a system of streams existed before the lake 

 beds were laid down. It would naturally be expected that many of 

 these should carry gold, and it is indeed probable that if the lake beds 

 were removed we should find auriferous gravels on the bed rock along 

 these lines of old stream courses. But it is extremely unlikely that a 

 random bore hole would strike any of these deposits. They could be 

 found only by prospecting along the margin of the lake beds and fol- 

 lowing down any rich stratum that might be found. In this case, 

 however, the cost and difficulties of mining would be very great. 



Olivine-basalt (dolerite). One mile above Idaho City, on the north 

 side of Moore Creek, there are peculiar outcrops of a black or dark- 

 green very tough rock, weathering in rounded outcrops, which are 

 commonly referred to as "nigger heads." Decomposing, they yield 

 a dark-red, yellow, or green clayey soil. Gravel benches rest upon 

 this rock, which apparently forms an intercalated bed up to 100 feet 

 thick in the lake beds. The sheet lies flat at the point indicated, 

 crosses Moore Creek in a narrow strait 2 miles above the city, and 

 then, probably being tilted, rises to elevations of 400 feet above the 

 creek near Pine Gulch. Here it is evidently separated by a fault 

 from the granite. The same rock is found again in nearly every one 

 of the small gulches entering Moore Creek opposite Idaho City, and 

 here it is covered by a considerable thickness of lake beds and is 

 exposed only in the bottom. It is a medium-grained, dark-green 

 rock, with abundant scattered crystals of greenish-yellow olivine. 

 Under the microscope it is seen to be a coarse olivine-basalt (dolerite), 

 and to consist of large phenocrysts of olivine and small crystals of 

 magnetite as the earliest product of consolidation. There is, further, 

 a large amount of violet-brown augite in large anhedral individuals, 

 forming a sort of base, in which are embedded the irregularly distrib- 

 uted laths of a basic feldspar (labradorite). The olivine decomposes 

 to brownish-red products, also to serpentine; the augite to chlorite, 

 with beautiful radial structure. The structure of the rock is really 

 that of a diabase. This rock was evidently poured out on the surface 

 as a lava at the time of deposition of the lake beds. 



THE VALLEY OF GRIMES CREEK. 



Configuration. Four miles below Centerville the canyon widens to 

 a broad valley with gentle slopes, similar to that of Moore Creek, 

 extending in an east-northeasterly direction for 11 miles. Above this 

 Grimes Creek makes a sudden bend, and, separated only by a low 

 ridge from the deep canyon of the Payette River, finally reaches its 



