98 



University of California Publications. 



[Geology 



ing the diastrophie record. Unlike the Tertiary river gravels 

 of the western slope of the Sierra Nevada, practically no work 

 of mining development has been done in this eastern channel, 

 though gold is found in some amount. Hence many of the char- 

 acteristics must await a more favorable time for their determina- 

 tion. The maximum width of the gravels is estimated to be 

 about 300 yards, but excessive faulting masks the dimension. 

 The depths where best preserved is at least 500 feet, and may be 

 much more, for exact data is not to be obtained. The gravels 

 range from coarse, with boulders a foot in diameter, to fine 

 gravel that would pass a 1-inch screen. The form of all is very 

 rounded and indicates a long passage in the waters of the old 

 river. Thus far search has revealed worn fragments chiefly of 

 the old metamorphic rocks, with some porphyritic boulders that 

 may belong to the same Jura-trias formation, or may be derived 

 from the diabase-porphyrite series. Gabbro-like rocks and grano- 

 diorite are also found, but are badly decomposed. Of the un- 

 doubted metamorphic rocks quartzite boulders, often a quartzite 

 breccia, are in great abundance. Schists and hornfels also are 

 present. Near Lakeview there is a large amount of nearly pure 

 iron ore in that portion of the channel mined years ago. Both 

 magnetite and hematite are present, in boulders up to a foot in 

 diameter. Some little quartz in veinlets is present. Crystalline 

 structure is well developed, and causes the worn surface to ex- 

 hibit complex etched figures. A few small boulders of a schistose 

 rock are found containing hematite and magnetite in small 

 stringers and irregular masses. The matrix is amphibolitic, but 

 very fine grained. The source of the iron ore is, therefore, prob- 

 ably from the contact zone of the granodiorite and schists. The 

 gold is fairly coarse, averaging the size of grains of wheat. The 

 writer has been informed by those who have been interested in 

 the mining of the gravels near Franktown that about $160,000 

 has been extracted in a crude way. The gravels are partly cov- 

 ered by later lavas. It is yet impossible to determine absolutely 

 the direction of flow of this old river. The only criterion now 

 of much value is the relation of the composition of the gravels 

 to the rocks of the surrounding country. The metamorphic 

 areas yet in evidence show much quartzite west of Lake Tahoe, 



