Hillebrand, Turner and Clarke — Roscoelite. 457 



also be the augite-porphyrite tuff associated with it. The ser- 

 pentine is later than the Calaveras formation and earlier than 

 the granodiorite, which is probably late Jurassic or early Cre- 

 taceous. It will be observed that all of the localities are at or 

 near the contact of an intrusive granodiorite mass with the sur- 

 rounding rocks, chiefly sediments and older lavas. This suggests 

 that the mineral may be regarded as in some way due to the 

 mineralizing solutions accompanying the intrusion of the gran- 

 odiorite. However, the quartz veins clearly fill fractures 

 which formed in the granodiorite and associated rocks after the 

 consolidation of the granodiorite ; consequently the deposition 

 of the quartz and the associated gold and roscoelite must have 

 been also subsequent to the consolidation of the granodiorite. 

 The following are the localities reported by Mr. Kimble. 

 They are all in Eldorado County, from 8 to 15 klm northwest of 

 Placerville. 



Locality 1. Thompson Hill on its northeast slope about 2 , S klm 

 southeast of the Stockslager mine. — There are here fif- 

 teen or more small seams of quartz having a strike of north 

 of west. As these seams pass through the contact of the 

 granodiorite with the greenstone (augite-porphyrite ?) they con- 

 tain rich spots of gold and roscoelite. The seams of quartz 

 pass on through the northeast point of Thompson Hill and 

 come into the granodiorite again. 



Locality °2. Stockslager mine on Granite Creek. — Mr. 

 Lindgren informs me that the vein of the Stockslager mine is 

 in a narrow wedge of metamorphic sediments of the Calaveras 

 formation. Immediately east of this wedge is granodiorite, 

 and immediately west is serpentine. Mr. Kimble states that 

 there is here but one vein of quartz. This passes from the 

 granodiorite into the narrow wedge of the Calaveras formation. 

 It does not penetrate the serpentine.. As previously stated, it 

 was at this locality that by far the larger part of the roscoelite 

 was obtained, including that analyzed by Dr. Hillebrand, 



Locality 3. South slope of spur about #-7 klms north of the 

 milage of Lotus. — In micaceous slate, which is part of a con- 

 tact metamorphic zone of the Calaveras formation. The exact 

 locality is about 500 feet east of the contact. It has never 

 been demonstrated, but Mr. Kimble thinks that this quartz vein 

 extends southeast into the granodiorite. 



Locality 4- Clark Mountain on its east slope at the contact 

 of the Calaveras formation and the granodiorite.— The occur- 

 rence here is identical with that at the Stockslager mine. 



According to Kimble no roscoelite has ever been found 

 where the seams are in granodiorite, and the latter is not 

 altered; but some has been found at the contact of the quartz 

 with serpentine in localities 2 and 4. All of the quartz seams 

 at localities 1, 2, and 4 dip southwest and have pockets of gold. 



