Fairbanks.] 



Geology of Point Sal. 



45 



by F. Leslie Ransome,* and to others noted by the writer, which 

 are supposed to be nearly contemporaneous with the older uncrys- 

 talline rocks of the Coast Ranges. 



Microscopic Petrography. — As was to be expected from a study 

 of these rocks in the field, the microscope shows that the different 

 structural facies blend into each other. The rocks have undergone 

 considerable alteration, and, even in the case of the comparatively 

 fresh exposures at the base of the cliffs, it is often difficult to tell 

 exactly what the original mineralogical composition was. 



The specimens of basalt taken for microscopic study are almost 

 exclusively from the sea cliffs at the western end of Point Sal. But 

 one specimen was obtained from the highest part of the ridge from 

 which a slide could be prepared, and it seemed to be so similar to 

 the basalt at the end of the Point, that there can be no doubt of 

 the geological unity of the whole area of the basalt. In the hand 

 specimen, it appears as a dense rock with the rare occurrence of 

 porphyritic constituents. Under the microscope there are found, in 

 a few slides of the basalt, scattered pyroxene crystals of the first gen- 

 eration. It is an almost colorless, non-pleochroic augite and quite 

 fresh in appearance. The groundmass consists of small lath feld- 

 spars imbedded in a matrix of varying character. In the fresher 

 specimens, this consists partly of a finely granular substance polar- 

 izing like augite, and probably referable to that mineral, and partly 

 of brownish to greenish interstitial matter having the properties of 

 glass. Nearly all the specimens contain a greater or less amount of 

 green fibrous aggregates. The individuals of these aggregates are 

 arranged radially, so that they show a stationary dark cross as the 

 stage of the microscope is rotated. They are strongly pleochroic, 

 and polarize in yellow, green, and bluish tints, belonging undoubt- 

 edly to some variety of chlorite. Many sections show a tendency of 

 the feldspars to separate in two generations, but in none were large 

 crystals observed. The larger of the lath feldspars in the ground- 

 mass are sometimes polysynthetically twinned, but more commonly 

 are combinations of two simple individuals. They generally have 

 somewhat ragged outlines, particularly at the extremities. The 



*Bull. Dept. Geol., Univ. Cal., Vol. I, p. 71. 



