222 



University of California. 



[Vol. 3. 



to call the attention of petrographers to its character and mode 

 of occurrence, in so far as these can at present be ascertained. 



The locality lies in a region of peculiar geological interest, 

 occupying the northeast corner of the Bidwell Bar quadrangle as 

 mapped by H. W. Turner of the U. 8. Geological Survey. It is 

 on the lower flank of Spanish Peak about two miles due east of 

 the summit, near the southwest margin of a broad belt of 

 peridotite. It is thus near the line of faulting to which the bold 

 eastern scarp of Spanish Peak owes its origin. 



The lower flanks of the mountain are traversed by numerous 

 small creeks which cut up the surface into a succession of alter- 

 nating ridges and gulches. The corundiferous rock occurs on 

 one of these minor ridges, and the corundum was first discovered 

 by Mr. Edman as float in the adjoining gulch, and by him 

 traced to its source. 



The Peridotite Cut by Corundiferous Dyke. — The corundifer- 

 ous rock at this locality occurs in the form of a white feldspathie 

 dyke cutting a rock which is referred to in several of Turner's 

 publications on the geology of the Sierra Nevada. In the 14th 

 annual report of the U.S.G.S. he says; "The largest single 

 dyke of serpentine which is known to have been originally a 

 peridotite or pyroxenite occurs in the northern end of the range 

 in Sierra and Plumas counties. . . . This serpentine dyke has a 

 width, where it is crossed by the middle fork of the Feather 

 River, of more than 3 miles." In the 17th annual report he 

 gives the following petrographieal note concerning this rock: 

 "Locality: one and one-half miles west of Spanish Ranch post- 

 office. Microscopically this is an apparently fine-grained purplish 

 and green rock, evidently in part serpentine. Microscopically, 

 the structure is coarse granular, and the rock is largely olivine, 

 in rather large anhedrons, intersected by a network of cracks, 

 which cross at all angles, and along these cracks serpentine is 

 forming. Fibrous serpentine and tremolite occur between the 

 olivines evidently as alteration products. Associated chiefly 

 with the serpentine are black streaks of magnetite in aggregates 

 of minute grains. Chromite or picotite may be present but was 

 not observed." In a later publication* he modifies this descrip 



*The Bidwell Bar Polio, 1898. 



