200 



University of California. 



[Vol. i. 



Whatever may be the source of the silica forming the cherts 

 and jaspers in other portions of the Coast Ranges, on Angel Island 

 it appears unnecessary to call upon any hypothesis of a general 

 secondary silicification to explain their origin. It is a more simple, 

 and. in the light of the foregoing facts, a more probable supposition 

 that they were originally siliceous deposits, the silica being derived 

 largely from organic remains and varying in amount in different 

 portions of the series, both vertically and horizontally, with the 

 former abundance or scarcity of the siliceous organisms. When 

 these were few, the non-siliceous components of the deposit would 

 predominate and furnish a dark matrix for the preservation of the 

 radiolaria. When, on the other hand, the latter were very abun- 

 dant, the deposit would be largely opaline silica, in a condition sus- 

 ceptible to solution, and the organic structure would become oblit- 

 erated. 



Both the sandstone and the chert have been locally metamor- 

 phosed by invading eruptive rocks, but these alterations will be 

 described under their appropriate heads. 



THE FOURCHITE. 



Occurrence. — It is only for want of a better name to apply to the 

 eruptive rock" now to be described, that the writer has adopted the 

 above designation, being confronted with the difficulty of naming a 

 rock that is unquestionably a geologic unit, and at the same time 

 exhibits facies which, when separately considered, would receive 

 distinct petrographic names. It will appear that the name is appli- 

 cable only to the deeper-seated and more central portions of the 

 larger rock masses, and could not be applied to their peripheral 

 modifications, nor yet to the smaller isolated apophyses were they 

 considered as distinct occurrences. There has even been some 

 doubt as to the fitness of applying the name "fourchite" to the 

 main igneous masses, as they appear to present certain differences 

 from the typical fourchites described by Williams;* on the other 

 hand, they would seem to fall most readily into this group, and the 

 observed differences are not of a sufficiently pronounced character 

 to justify the introduction of a new rock name. 



"■The Igneous Rocks of Arkansas, Annual Rept. Geol. Surv. of Ark. for 



1S90. Vol. II, p 107, et seq. 



