Fairbanks.] 



Geology of Point Sal. 



the microscope it appeared to consist of an aggregate of minute 

 and strongly polarizing grains of chalcedonic quartz. The circular 

 areas did not appear as numerous as in the hand specimen, and were 

 only faintly distinguished by clearer polarization. Numerous speci- 

 mens were observed in which there was a confused mixing of the 

 clear flint with that of a dull color. These two kinds were related 

 in such a manner as to preclude the idea that it was due to any 

 original difference but was suggestive of secondary change. The 

 clear portions seem to have replaced the other in an irregular manner 

 along the lines of stratification, shreds of one projecting into the 

 other. Under the microscope the change in appearance is seen to 

 be due to the change from an amorphous silica to a crystalline vari- 

 ety, the dull portion of the rock consisting almost wholly of non- 

 polarizing silica, while the clear portion polarizes. This is not a 

 local character but has been observed by the writer in many por- 

 tions of-the Coast Ranges where the Miocene flints occur. This 

 change must be associated with that which led to the deposition of 

 the chalcedonic silica in the little veins which traverse the flints. 



Several sections prepared from the light yellow or white shale 

 gave interesting results. In these cases the greater part of the surface 

 shown consisted of a granular isotropic aggregate. Mingled with 

 this were polarizing grains and round spots filled with a brightly 

 polarizing silica similar to that in the first section. A number of 

 the circular areas exhibited an organic structure, and are referable 

 probably in part to foraminifera and in part to radiolaria. There 

 were a few clear, rectangular, elongated areas of small size which 

 polarized strongly, and extinguished parallel. There were also a 

 few irregular areas resembling in outline fragments of volcanic 

 glass, but this could not be decided with certainty. These were 

 the only indications of volcanic material in the rock. Not only is 

 the amount of material small, which could with any certainty be 

 referred to volcanic origin, but there were detected no particles of 

 clastic origin. 



Chemical Character. — A specimen of the white porcelain-like 

 shale was taken for analysis* (I). It is easily scratched with a 



* All analyses in this paper, unless otherwise stated, are by the author. 



