PETROLOGY IN AMERICA. 479 



developed while the outlines of the radiolaria are still 

 recognisable. Glaucophane-schist has also been produced 

 at the contact of the sandstone with the serpentine. The 

 blue amphibole mineral in these various rocks seems to be 

 in general true glaucophane, though it is possible that 

 crossite or other varieties may also occur. These observa- 

 tions are evidently of importance with reference to the 

 origin of the glaucophane-schists in general of California, 

 and perhaps of other districts. Lawson has expressed a 

 confident opinion that all the glaucophane-, hornblende-, and 

 mica-schists in the Coast Ranges are products of contact- 

 metamorphism by basic eruptive rocks. 



Some other points in the geology of Angel Island are 

 of interest in connection with an earlier paper by the same 

 author upon the eruptive rocks of Bonita Point, the northerly 

 horn of San Francisco Bay (35). The strata here still 

 belong to the San Francisco Sandstone, and include beds 

 of "red jasper" corresponding with the radiolarian cherts 

 mentioned above. The chief igneous rock is a basalt with 

 what is for convenience called a spheroidal structure. The 

 distinct portions into which the mass divides are, however, 

 not spheroids but rather bale-like and pillow-like bodies, 

 which give the effect of having been squeezed against one 

 another while in a plastic state, a description which would 

 apply to part of the " fourchite " of Angel Island and to 

 some " spheroidal " basalts and diabases in other regions. 

 The author expresses the opinion that the rock was poured 

 out as a somewhat viscous lava (the viscosity being ascribed 

 to the 2 or 3 per cent, of titanic acid present), successive 

 sluggish out-pourings being piled upon one another, 

 becoming sometimes spheroidal by rolling and sometimes 

 lenticular by flattening. In addition to the basalt, there is 

 at Bonita Point an intrusive diabase containing iddingsite, 

 which Ransome regards as probably a pseudomorph after 

 olivine. This has in piaces a spheroidal structure of a 

 somewhat different kind attributed to a kind of rlow-breccia- 

 tion. The "fourchite" of r\ngel Island shows in one 

 intrusion a spheroidal structure allied to brecciation, and in 

 another the peculiar structure described in the basalt. The 



