LAWSON "] 



Tai.acheJ 



The Berkeley Hills. 



439 



existed, since all formations above the basalt in this period have 

 been removed by erosion in the sections with which we are here 

 concerned. In connection with this apparent defect in period IV 

 it is worthy of note that to the north of Berkeley a sheet of soda 

 rhyolite occurs, which has been described by the junior author of 

 this paper.* This North Berkeley rhyolite is less than half a mile 

 from the northwest corner of the area represented on the accom- 

 panying geological map. It is known to be pre-Campan in age, 

 and it is, therefore, entirely possible that it stratigraphically occu- 

 pies the vacant place in period IV. In period V there is included 

 a fivefold sequence. The first three members conform to the 

 normal sequence of the first three periods. The remaining two 

 members might be regarded as the upper two-thirds of a sixth 

 period. 



This remarkable periodicity may possibly be accidental. If a 

 larger field were under consideration it might, perhaps, be found 

 that rhyolite sometimes succeeded andesite instead of always fol- 

 lowing basalt as it does in these five periods, or that andesite 

 occasionally succeeded basalt instead of preceding it. If any gen- 

 eral inference were to be based upon this sequence, that inference 

 would be greatly weakened by these possibilities of accident and of 

 error due to the limitations of the field of observation. The 

 sequence is, however, not so employed. It is regarded merely as 

 contributary data to the general question as to whether there is or 

 is not any normal or natural succession of volcanic lavas. This 

 general question has been before penologists ever since the publi- 

 cation of Richthofen's classic memoir on a " Natural System of 

 Volcanic Rocks," f and data have been gradually accumulated 

 upon which an answer to the question will be eventually based. 

 The sequence in the Berkeley Hills is, therefore, but a contribu- 

 tary fact to the many which must be considered before a reliable 

 scientific induction can be drawn. Whether it is an expression of 

 a general law of succession of volcanic rocks or is due to local and 

 special causes will depend on the extent to which it harmonizes 

 with sequences worked out in numerous other localities. The 



*This Bull., Vol. I, No. 2. 



tMem. Calif. Acad. Sci., Vol. I, p. 36. 



