120 CALIFORNIA ACADEMY OF SCIENCES. 



maps as angite anclesite is divisible into two separate erup- 

 tions of different dates. 



I affirm that the structural relations and the succession of 

 rocks as set forth in my memoir, is substantially correct. 

 In particular, the pyroxene andesite, diabase and diorite 

 exposed in the Sutro Tunnel, do not form one continuous or 

 contemporaneous rock mass, as would be necessary if this 

 exposure were to lend any support to the hypothesis of pro- 

 gressive increase of crystallization. On the contrary, these 

 rocks constitute at least three distinct eruptions, separated 

 by long time-intervals. 



I consider it possible that the quartz porphyry, although 

 of greater age than the andesites, may have been erupted in 

 early Tertiary times, but this I think unlikely. 



Though there may be local segregations of plagioclase 

 in the quartz-porphyry, five new separations by the Thoulet 

 method show that it is substantially an orthoclase rock. 



I think it possible, but improbable, that the black dike is 

 basalt. In the present state of science, an absolute decis- 

 ion on this point is impossible. 



The remainder of the conclusions stated at the close of 

 Messrs. Hague and Iddings paper, I deny. 



I conclude also that valuable as is the study of collections, 

 inferences from them may easily be pushed too far; and 

 that it is impracticable to elucidate the structure of a com- 

 plex region from collections, however extensive. 



Office of the U. S. Geological Survey, ) 

 San Fkancisco, December, 1885. j 



state that they have revised them by comparison with the original records 

 so far as possible. This was evidently by no means superfluous. They 

 have made four changes in V, which seems a large number of misprints in a 

 single analysis. In VII, they have made only one correction; but the orig- 

 inal record of this analysis must be faulty, since the sum of the items, as 

 they give them, still fails to tally with the total. While the effect of minute 

 variations of composition seems beyond question well marked, it is not en- 

 tirely clear what effects should by expected from high 'pressures, the consid- 

 eration of which, at once brings up the perplexing question of the relative 

 dynamical influence of absolute stress and stress-difference. 



