THE WASHOE ROCKS. 95 



the physical conditions through which a mass of readily 

 ascertainable chemical composition has passed. Hence, in 

 the present state of ignorance concerning the effects of high 

 temperatures and pressures, the most rational method is to 

 study and record every peculiarity of every occurrence and 

 every perceptible difference between rocks. When at some 

 future time the causes of the observed effects are well 

 known, it will be easy to ignore distinctions which are in- 

 significant. If all traceable distinctions are not now pre- 

 served, however, it will then be necessary to trace them out 

 lest significant differences should be neglected. It has never 

 appeared to me, for example, that a distinction between 

 pre-Tertiary and Tertiary eruptions was a natural one, but I 

 regard it as an artificial substitute which it would be unwise 

 to abandon, at least until some available natural principle 

 distinguishing little eroded from deeply eroded rocks is 

 discovered and thoroughly established. 



Eocks can, of course, never be classified with the sharp- 

 ness of minerals. Kocks are essentially mixtures and 

 therefore pass over into one another insensibly. The won- 

 der is, that rocks not only conform in some degree to a 

 system, but that certain lithological types exhibit such an 

 extraordinary persistence, being met with at the most re- 

 mote quarters in typical development. While the very 

 nature of the case thus excludes a rigid classification of 

 rocks, observation clearly indicates the possibility of reduc- 

 ing them to a natural system. 



THE PORPHYRITIC PYROXENE EOCKS. 



The Issue. — Those who are familiar with the points at 

 issue between Messrs. Hague and Iddings and myself, will 

 readily see that the main subject of controversy is the pyrox- 

 enic rocks. ^ In my memoir on the Comstock, I claimed 



Note '. — Mr. VV. Cross' paper on hypersthene andesites was published after 

 my lithological discussions of the Washoe rocks was ready for the press, and 

 too late for a revision. The Washoe pyroxenic rocks contain much hyper- 

 sthene, although the quantity of augite usually exceeds that of hypersthene. 



