94 CALIFORNIA ACADEMY OF SCIENCES. 



said to afford conclusive proof of it unless my conclusions 

 as to structure and succession were lirst overtliro\yn. 

 This demolition they have somewhat ruthlessly attempted. 

 During the past season I have re-examined the Washoe 

 district with their paper in hand, but without being able to 

 detect any substantial error in my former results. I also 

 gathered many new facts concerning the relations of the 

 rocks and, much as I regret being drawn into a controversy, 

 it seems needful to call attention to these as well as to 

 arguments not presented, or imperfectly presented in my 

 former report. I shall be as brief as possible and deal only 

 with the, more essential points, being unwilling to con- 

 tribute an unnecessary word to controversial literature. 



SOME GENERAL PRINCIPLES. 



IBefore proceeding to points which are in dispute, I desire 

 to state certain principles concerning which, so far as I 

 know, Messrs. Hague and Iddings would wholly agree 

 with me. Given the chemical composition of an eruptive 

 magma; the mineralogical results are dependent solely on 

 the physical conditions to which it is subjected. It is not 

 a question therefore, whether if similar magmas are sub- 

 jected at different times to similar temperatures and press- 

 ures similar mineralogical and lithological results will 

 ensue, but whether at different geological eras the physical 

 conditions attending the cooling of eruptive masses have 

 been substantially identical. That this has sometimes been 

 the case will scarcely be denied. The problem with which 

 geologists have to deal, however, is not precisely that just 

 stated, for, since the earlier formations have been deeply 

 eroded while the degradation of comparatively recent rocks 

 is as a rule correspondingly small, upper portions of more 

 recent eruptions have to be compared with lower portions 

 of more ancient eruptions. The lithological problem is 

 thereby greatly complicated. 



The main purpose of lithology, to my thinking, is to trace 



