DIKE ROCKS ON ROCK CREEK. 711 



hornblende and biotite, all embedded in a reddish ground mass. 

 Under the microscope the feldspars are shown to be a plagioclase of 

 medium basicity. The hornblende and the biotite are of normal char- 

 acter, while the groundmass is microcrystalline, consisting of quartz 

 and unstriated feldspar. The structure of the groundmass often is 

 approximately micropoikilitic. In certain of these porphyrites horn- 

 blende is very abundant and occurs also as small prisms embedded 

 in the groundmass. 



Another kind of porphyry contains quartz as rounded phenocrysts, 

 and in this variety no quartz occurs in the groundmass. At the cross- 

 ing of Rock Creek the dike is wide and fresh and the rock is of 

 somewhat different character. It is dark green and fine grained, with 

 feldspar prisms up to 8 mm in length and a few black, shining crystals 

 of hornblende. The feldspar is chiefly labradorite in sharp, short 

 . prisms and very fresh. The crystals are of all sizes, grading down 

 to those which form a part of the groundmass. Augite occurs as 

 idiomorphic crystals, many of them decomposed; a little hornbler.de 

 is also present, and magnetite is quite abundant. Between the closely 

 crowded feldspars lies very little groundmass of quartz and unstriated 

 feldspar. The structure of this rock is intermediate between holo- 

 crystalliiie porphyritic and panidiomorphic granular. The rock is 

 thus an augite-diorite-porphyrite, closely connected with the lampro- 

 phyric dike rocks. 



At the Dynamite claim, near Pearl, dense, dark-green dikes appear, 

 consisting of clouded and altered feldspar in lath-like form, between 

 which lie the decomposed ferromagnesian silicates replaced by chlo- 

 rite and epidote. This rock has somewhat the appearance of an 

 altered, fine-grained diabase. Scattered dikes occur in other parts of 

 the district, one large dike cropping on the road to Marsh, 1 miles 

 north of Pearl. On the whole, the dikes may be said to be most 

 abundant for a distance of half a mile north and half a mile south of 

 the main belt of porphyritic rocks above described. 



THE PAYETTE FORMATION. 



The loose sandstones of the Payette formation (early Neocene) are 

 laid down upon the very irregular granitic surface and begin at the 

 western edge of the Willow Creek district, extending far westward. 

 Sandstones and fine gravels of the same age are also found high up in 

 the vicinity of Prospect Peak, and cover, in fact, the pass leading from 

 Willow Creek to Boise. A branch of the Payette sandstones extends 

 in a northeasterly direction as far as Horseshoe Bend. The Payette 

 formation locally contains gold placer deposits, as at Church's ranch, 

 at the southern edge of Marsh Valley. The gold in these placers has 

 doubtless been washed down from quartz veins on the northern slope 

 of Crown Point Hill. Other placer deposits occur 1 mile southwest 

 of Marsh in the sands and conglomerates of the Payette formation. 



