44Q 



University of California. 



[Vol. 2. 



sequence at present known, as worked out for example by 

 Brogger* in the Christiania region, by Iddingsf in the Yellowstone 

 Park, by GeikieJ for Great Britain, and more recently by Spurr§ 

 in the Great Basin, are but partially harmonious, and it is evident 

 that many more observations will be necessary in favorable locali- 

 ties, where the fortuitous or accidental sequence can be discrimi- 

 nated from the normal succession, before an acceptable general law, 

 if there be any such law, can be formulated. The most satisfactory 

 working hypothesis that has been advanced up to the present time 

 is that announced by Iddings, || viz., that "the general succession is 

 from a rock of average composition through less silicious and more 

 silicious ones to rocks extremely low in silica and others extremely 

 high in silica, that is, the series commences with a mean and ends 

 with extremes." 



Now the various periods of our local succession in the Berkeley 

 Hills are clearly in harmony with this hypothesis, and to that extent 

 support it. To the extent that they are harmonious, moreover, we 

 have some measure of probability that the local sequence is an 

 expression of the normal or general succession of volcanic rocks. 

 It is to be noted, however, that in each of the periods of our 

 sequence, there is an immediate passage from the andesite, or rock 

 of intermediate composition, to the basalt, the most basic, without 

 transitions, and similarly an immediate change from basalt to rhyo- 

 lite. Moreover, the change is always from andesite to basalt and 

 then to rhyolite. In this respect the succession is more definite 

 and more clear cut than is implied in Idding's hypothesis. Fur- 

 ther, the periodicity or recurrence of the ternary sequence is more 

 remarkable than the mere harmony of the periods with that hypoth- 

 esis; and the periodicity minimizes the possibility that the har- 

 mony might be a mere coincidence. 



-Q.J. G. S., Vol. L, pp. 15-38. 



I The Origin of Igneous Rocks, Bull. Phil. Soc. Wash., Vol. 12, p. 144. 

 tQ. J. G. S., Vol. XLVIII, 1892. 

 2Jour. Geol., Vol. VIII, p. 62r, 1900. 

 || Loc. cit. 



