Ransome.] 



Eruptive Rocks of Point Bonita. 



ii3 



to flow onward, and consequently to separate itself from the cooling 

 shell, it being an example in miniature of the lava tunnels of Hawaii. 

 Where this separation was taking place, the lava would be under 

 diminished pressure, and the included vapors would expand into 

 vesicles, as was found to be the observed fact.* 



As regards the probable position of the former center of volcanic 

 activity, — the coarseness of the ejected fragments composing the 

 pyroclastic rock, the general lack of regularity and continuity of the 

 beds, the spheroidal and bale-like forms of the basalt, and the occur- 

 rence of both surface and intrusive rocks, — all indicate that the vol- 

 canic focus was not far away, and probably lay to seaward at some 

 little distance off the present coast. 



The writer's grateful acknowledgments are due to Prof. A. C. 

 Lawson, under whose supervision the foregoing investigations were 

 conducted, for his unfailing kindness in regard to advice and sug- 

 gestions. 



Geological Laboratory, University of California, Nov. 1, f8<?J- 



* Since the foregoing was written, the writer's attention has been directed 

 to a paper by Messrs. Fox and Teall (Radiolarian Chert from Mullion Island, 

 Q. J. G. S., Vol. XLIX, 1S93, p. 211) in which a " greenstone" showing rolled 

 and ropy masses is described in connection with radiolarian cherts at the 

 Lizard. This occurrence seems to present many interesting parallelisms with 

 that of the Bonita rocks, particularly in regard to the difficulty found in 

 deciding as to the intrusive or surface character of the "greenstone," — a diffi- 

 culty which the authors suggest may be removed by supposing the lava to have 

 been intruded between the sheets of the chert, very near the surface of the 

 sea-bed upon which they were being deposited. 



