ransome.] Eruptive Rocks of Point Bonita. 105 



above the beds of pyroclastic rock (see section B' B"), it loses its 

 most important title to being considered a surface rock, and may 

 possibly belong to the diabase and be intrusive, notwithstanding its 

 typical spheroidal structure. That such structure is not wholly 

 incompatible with the idea of intrusion has been shown by Greg- 

 ory.* It seems most probable, however, that this also was a 

 surface flow which succeeded and covered the agglomerate, the dia- 

 base subsequently being intruded between the members of the sur- 

 face series, as shown in section B' B", the fragments of jasper being 

 in this case caught up by the flow, and derived from a lower hori- 

 zon than that of the bed mapped. 



Chemically the two rocks are seen to be practically identical, 

 there being as much variation between rocks undoubtedly of the 

 same eruption as between those of different age. The analyses 

 indicate that the microscopic differences observed in the rocks are 

 mainly conditioned by physical causes, and that the absence of a 

 mineral in one rock is not by any means an indication that the 

 elements required for its formation are lacking in the general 

 magma, or that it would not have developed under somewhat dif- 

 ferent conditions of solidification. f The presence of augite in the 

 diabase and its absence in the spheroidal basalt can be regarded as 

 due only to physical conditions of cooling, and is analogous to the 

 fact recorded by Professor LawsonJ in regard to the carmeloite of 

 Carmelo Bay, — augite there occurring in the plug rock but not in 

 the surface flows. 



It is unfortunate that the generally decomposed or altered con- 

 dition of the spheroidal basalt prevented a more thorough chemi- 

 cal comparison with the diabase, only one analysis of the former 

 being made, and that of a somewhat local phase. 



The considerable proportion of titanic oxide is noteworthy, and 

 in view of its supposed influence upon the fusibility of rock magmas- 

 as pointed out by Dr. Becker, § it is natural to infer that it plays 

 * Loc. ext., pp. 58, 59. 



tj. P. Iddings, The Crystallization of Igneous Rocks, Bull. Philos. Soc, 

 Wash., Vol. XI, p. 92, and The Mineral Composition and Geological Occur- 

 rence of Certain Igneous Rocks in the Yellowstone National Park, ibid., p. 213- 



% Loc. cit., p. 44. 



'i See H. W. Turner's paper on the Geology of Mount Diablo, Bull. Geol. 

 Soc. Am., Vol. 2, p. 386. 



