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University of California Publications in Geology [Vol. 11 



sents a pebble which shows some of these curving veinlets on its sur- 

 face. At times the massive metamorphosed chert near the contacts 

 of igneous rocks is cut by a great number of veinlets of quartz 

 arranged in a complex network (plate 31c). 



Spherulitic Cherts 



Thin sections of cherts taken near igneous contacts may show a 

 development of spherulites consisting of radial aggregates of fibrous 

 chalcedony. These show a black polarization brush between crossed 

 nicols (plate 30, f). The extinction is parallel to the fibers in all cases. 

 Two varieties of chalcedony are present : one shows a negative elonga- 

 tion (chalcedonite) , while the other, less commonly present, shows a 

 positive elongation (quart sine) . The spherulites may be very abun- 

 dant, making up the larger part of the rock. Generally they are not 

 large enough to be visible in hand specimen, but in certain instances 

 they are as large as peas. 



Under the name "Kinraidite, " D. B. Sterrett 15 described a variety 

 of this spherulitic chert in which the spherules were unusually well 

 developed. This rock is found in Marin County and also at the Cliff 

 House in San Francisco at the contact with, and as inclusions in, the 

 intrusive basalt and diabase. In most cases the spherulites are red in 

 color and are imbedded in a matrix of silica which is green, red, brown, 

 or yellow. The spherulites range from microscopic dimensions up to 

 an inch or more. They show a fibrous radial structure and are com- 

 posed of chalcedony with a positive elongation. 



Fusion and Assimilation op Cherts 

 In many exposures of the intrusive ellipsoidal basalt, as at Hun- 

 ter's Point, in the sea cliffs in Marin County, and at Mount Diablo, 

 blocks of chert may be found included in the basalt near its contact 

 with chert masses. These blocks have been rendered massive, and 

 otherwise altered by the heat of the intrusive in the manner outlined 

 above. In certain instances there is also clear evidence of fluxing of 

 the chert in the basic magma. 



Blocks showing this action vary in size from twenty to thirty feet 

 in diameter down to only a foot or so. Their central portions consist 

 of highly colored, vermilion or orange chert, unchanged save for their 

 recrystallization under the action of the intrusive. Toward the outer 



is Mineral Eesources of the United States, pt. n, p. 870, 1910. 



