THE WASHOE ROCKS. llT 



throughout the region. This I deny. It is a region where 

 dikes should be expected, and to this fact I was full}' alive. 

 Mr. King, in his hypothetical section of the country, showed 

 several; and Mr. Church asserted that there were at least 

 twenty-five or fifty north and south dikes. Messrs. Stretch, 

 Eeade and I were constantly on the lookout for these im- 

 portant aids to geological interpretation and their almost 

 entire absence was repeatedly a matter of surprised com- 

 ment in my party. Except under unusual conditions, a dike 

 is recognizable with the utmost ease, and very few cases 

 could escape reasonably careful scrutiny. It is, of course, 

 possible to interpret variations in the state of decomposi- 

 tion and similar phenomena as dikes on superficial examin- 

 ation. This has often been done at Washoe, but these cases 

 do not stand the tests of careful study. At Steamboat, 

 among the same rocks, real dikes are not infrequent, and 

 the indications of their character are clear. 



Lithologiccd criteria. — I cannot but believe that Messrs. 

 Hague and Iddings, led away by the fascination of their 

 hypothesis, have unconsciously made a somewhat arbitrary 

 use of lithological criteria. Because the pyroxene andesite 

 strongly resembles the porphyritic diabase, they insist the 

 two rocks must be substantially of the same age, notwith- 

 standing the structural evidence to the contrary. Yet they 

 believe that pre-Tertiary eruptions are not, as such, distin- 

 guishable from later volcanic rocks. On general principles, 

 therefore, they would be satisfied with a moderate amount 

 of evidence of the diversity in the age of rocks which were 

 lithologically similar. In this particular case, however, 

 such proof would diminish the strength of their argument 

 for a relation between granulation and distance from a fixed 

 point. But lithological dissimilarity does not^stand in the 

 way of tlieir identifying rocks; for though only an infinites- 

 simal portion of the highly decomposed andesite of the Su- 

 tro Tunnel, possesses a banded structure, and though this 

 structure, common to various rocks, is the only point of 



