270 University of California Publications in Geology [Vol. ll 



While there is considerable variation in these shales they approach, 

 in many respects, the ordinary terrigenous shales. The high iron con- 

 tent is the most striking point of difference. Titanium was not deter- 

 mined in the samples represented by analyses n, hi, and v. A qualita- 

 tive examination of the material used for analysis iv gave no test for 

 titanium on boiling with hydrochloric acid and tin. 



METAMORPHISM OF THE CHERTS BY FRANCISCAN 

 INTRUSIVES 



The cherts have suffered more than the other members of the 

 Franciscan group through the intrusion of igneous rocks. On account 

 of their brittle nature and the numerous lines of weakness in the shale 

 partings, the cherts have presented zones which were favorable to the 

 intrusion of igneous rocks. 



Changes in Color and Structure 

 Near the contacts with the intrusive bodies, the cherts often become 

 massive through the disappearance of the shale partings. At first the 

 parting planes may be seen, though most of the shale has disappeared, 

 and the chert breaks easily along these planes. At a later stage, all' 

 trace of the bedding disappears; the rock becomes massive and will 

 split in any direction with equal ease. 



The colors also change. In many cases there is an increase in the 

 brilliancy of the color. Bright shades of vermilion are often devel- 

 oped, while in other cases the cherts take on bright orange, yellow, or 

 green colors. One hand specimen may show two or three different 

 colors, each color being separated into patches which contrast strongly 

 with one another. In other cases there is a bleaching of the colors of 

 the cherts. Some inclusions of chert in basalt assume a dirty white 

 or gray color, while others take on very pale shades of yellow or pink. 

 A single block of chert may show both kinds of color change. One 

 part of the block may be pale gray, green or brown, with streaks repre- 

 senting old parting planes; another may show bright reds or yellows, 

 being massive and without streaks. 



When examined with a hand lens, the bright vermilion contact 

 phase of the chert shows a peculiar curdled appearance, as though 

 the iron oxide had segregated into definite patches. In thin section 

 this is seen to be the case. One of the vermilion rocks taken at Hun- 

 ter's Point shows a matrix, composed largely of chalcedony in fairly 



