116 CALIFORNIA ACADEMY OF SCIENCES. 



gathered during the sinking of the shaft. By repeated 

 study of these specimens, and by comparisons between 

 them and decomposed portions of the hornblende andesite, 

 near the top of the shaft on the one hand, and with dia- 

 bases of the Sutro Tunnel level on the other hand, I 

 came to the conclusion that the hornblende andesite of the 

 surface was continuous from the top of the shaft to a point 

 about 250 feet above the tunnel level. At this point there 

 was a change in the character of the rock which corre- 

 sponded to a similar change in the tunnel about 100 feet fur- 

 ther east than the shaft. Through these points I drew the 

 contact after taking all available facts into consideration. 

 My determination of the width of the hornblende andesite 

 in the tunnel was neither a guess nor was it founded on any 

 theory, but was legitimately based upon the best observa- 

 tions which the nature of the case permitted. It is in entire 

 accord with the results of my more recent studies at Steam- 

 boat Springs, where as has been pointed out, the earlier 

 hornblende andesite is younger than one portion of the 

 pyroxene rocks and older than another portion. 



Dikes. — Messrs. Hague and Iddings claim that there is a 

 dike of later hornblende andesite in the pyroxene andesite 

 of the Sutro Tunnel. That for some distance the rock here 

 carries some mica is unquestionable. When I first detected 

 the presence of this mica, I believed that the later horn- 

 blende andesite was the last andesitic eruption, but the evi- 

 dence on this point gathered up to that time was not so 

 good as I desired. I should consequently have been glad 

 to consider this a dike, and during some sixty visits to the 

 tunnel, I examined this occurrence many times, but without 

 being able to make up my mind that there was sufficient ev- 

 idence to warrant the assertion of its intrusive character. 

 It is true that I did not regard mica as necessarily an unfail- 

 ing indication of one and only one rock, nor do I now. It 

 may be that this really is a dike. If so, it is a very obscure 

 case. They also maintain that dikes are very numerous 



