366 



University of California Publications. 



[Geology 



The writer inclines to the belief that this rock was originally 

 a facies of the Franciscan radiolarian chert. He has seen dis- 

 tinctly altered cherts that have a somewhat similar texture and 

 mineral appearance under the microscope. The Franciscan 

 cherts grade over insensibly into siliceous iron ores and in a 

 number of localities have associated with them deposits of man- 

 ganese dioxide. This would explain the high iron and low 

 alumina content and the association with manganese stringers. 

 Much or all of the soda and other oxides in part may have been 

 introduced during the metamorphism, as in the case of certain 

 crocidolite schists of the Coast Ranges which the writer has found 

 to have been derived from ferruginous cherts by a similar 

 process. 18 



Of the rocks described as associated with the veins the green- 

 stone (altered diabase) is the most abundant and the one most 

 commonly in contact with the veins in moderately altered con- 

 dition — especially towards the east end. On approaching the 

 central part of the zone of veination, however, the alteration in- 

 creases very greatly, the original pyroxenic constituents disappear 

 and the chief constituents are the new-formed amphiboles. The 

 old structures are entirely lost. In part we may refer to the 

 material as soda-amphibole schist. 



A still further alteration is caused by the leaching out of the 

 feldspathic constituents, leaving the rock in a more or less porous 

 condition, as occurs on the left side of the cut shown in plate 32. 



This rather porous rock near the veins may be thoroughly 

 impregnated with natrolite for a fraction of an inch or several 

 inches from the vein ; also it is in this rock that the spaces occur 

 covered with free-growing amphibole needles on which the natro- 

 lite groups are perched as already described. 



SEQUENCE OE EVENTS. 

 The field relations and lithologic characteristics indicate that 

 the rocks in which the benitoite-bearing veins occur are a detached 

 mass of the Franciscan series, showing both igneous and sedi- 

 mentary facies, that was included in the serpentine at the time 

 of its intrusion. 



is Louderback and Sharwood: Bull. Geol. Soc. Am., 18 (1906), abstract 

 p. 659. 



