1918] Davis: The Radiolarian Cherts of the Franciscan Group 275 



Alteration of Chert to Glaucophane Schist 

 Another peculiar alteration of the chert at contacts of serpentine 

 and basalt is a change to glaucophane schist. This has been described 

 by Ransome 10 from contacts of chert with ellipsoidal basalt and ser- 

 pentine on Angel Island. Here both chert and sandstone were altered 

 to glaucophane schist at contacts of both the serpentine and the ellip- 

 soidal basalt. Examples of similar action may be seen at a contact of 

 ellipsoidal basalt with chert in the region of Red Rock Canon, Santa 

 Barbara County. Here little sheaves of glaucophane have been pro- 

 duced in the midst of rather massive white cherts. 



The metamorphic rocks do not as a rule form a continuous border 

 around an igneous mass, but occur in small separated bodies along 

 the contact. Small isolated masses of glaucophane schist that occur 

 in areas where no igneous rock is exposed are best explained by the 

 idea that unexposed beneath the surface, there exists a body of 

 igneous rock, or that before erosion, there was a body of igneous rock 

 above the schist. In addition to the actually observed contacts, it is 

 a very common thing to find, in creek-beds, large boulders of massive 

 chert, altered by igneous action which show evidences of a change to 

 glaucophane schist. 



One boulder three feet in diameter, found in Perkins Canon at 

 Mount Diablo, is composed of radiolarian chert on one side while 

 the other side is glaucophane schist. The chert portion is composed 

 of red chert which has been somewhat altered ; the shale has largely 

 disappeared, but the partings are present, though intensely contorted. 

 They are cemented, so that the rock splits about as easily across former 

 parting planes, as parallel to them. The chert is traversed by some 

 quartz veins. 



In the portion altered to glaucophane schist, there is definite evi- 

 dence of the former bedding of the chert. The glaucophane schist 

 consists of alternating bands of hard and soft material. The softer 

 bands of less siliceous schist appear to represent the original shale 

 partings. The harder portions which are more siliceous represent the 

 former chert layers, and owe their hardness to the excess silica which 

 they contain. 



The border zone between the chert and the glaucophane schist is 

 rather narrow, but may be clearly distinguished everywhere. It is 

 marked by a space within which the chert is cut by numerous stringers 



is Bansome, F. L., The Geology of Angel Island, Univ. Calif. Publ., Bull. Dept. 

 Geol., vol. 1, p. 193, 1894. 



