156 H. IV. TURNER 



pegmatite, with less quartz, titanite, apatite, epidote, pyroxene, 

 and uralite. A rough calculation shows that this rock is com- 

 posed of about 64 per cent, of albite, 25 per cent, quartz, the 

 remaining 1 1 per cent, including pyroxene, titanite, apatite, epi- 

 dote and uralite. It is thus a true soda-granite. 



South of the locality at which 399 was collected, on Agua 

 Fria Creek, the soda-aplite is in sharp contact with granodiorite, 

 but there was no satisfactory evidence found of the relative age 

 of the two rocks. At the head of Owen's Creek, to the west 

 of Cathay Valley, there is better evidence of the age of the 

 soda-granite. The clay slates, which are pretty certainly of 

 Juratrias age, are here clearly metamorphosed by the granitic 

 rock. 



In Butte and Plumas counties white dikes are abundant in 

 metamorphic magnesian rocks, which are altered peridotites and 

 pyroxenites. These dikes are mostly composed of quartz and 

 albite, but in some muscovite is present. Analysis 725 is of a 

 specimen collected from a dike in serpentine on Grizzly Hill in 

 Plumas county. It is composed chiefly of spherulites of quartz 

 and albite, micropegmatite, and abundant muscovite, the latter 

 mineral chiefly in little rosettes. It has elsewhere * been sug- 

 gested that these dikes of soda-granite and aplite are in some 

 way genetically related to the peridotites and pyroxenites or 

 other basic rocks with which they are usually associated. 



The aplite dikes in the gneisses and associated granites, — In the 

 bed of the North Mokelumne and other points there are irregular 

 white dikes in the gneisses and associated granitic rocks. Some 

 of these dikes are of evenly granular texture throughout, and 

 may be called aplites ; others are banded. A chemical analysis 

 has been made of only one of these dikes, and this analysis 

 taken in connection with the microscopical examination indi- 

 cates that the rock is rich in soda, and hence the aplites in the 

 gneisses are placed with the soda-aplites. It is by no means 

 certain, however, that they are all alike in composition. Some of 

 these dikes contain garnets. The aplites in the gneisses and 



1 Bidwell Bar folio of the Atlas of the U. S. Geol. Surv. 



