240 University of California Publications in Geology [Vol.8 



Cananea. — At Cananea, Emmons' maps 40 show that the Capote 

 basin is a faulted syneline, the Cambrian quartzite and limestone 

 dipping down toward the Eliza fault. This fault which is estimated 

 to have a throw of 1000 feet is, according to Emmons and Lee, 41 later 

 than the formation of the ores. Emmons ascribed the ores chiefly to 

 magmatic waters emanating from the diorite porphyry which he sup- 

 posed intrusive. Lee 42 has since shown that the diorite porphyry is 

 an extrusive rock of the nature of a tuff "and as such could not have 

 produced contact metamorphism. " He therefore considers the ores 

 to have been formed by magmatic waters escaping from the Eliza 

 quartz-monzonite-porphyry. If we assume that the latter rock is 

 intrusive, then, before the development of the Eliza fault, the Cam- 

 brian strata must on their dip have abutted upon the porphyry ; and 

 the conditions here also would have been favorable for the convergence 

 of the meteoric waters upon the hot intruded mass and for circulation 

 upward on its margin. 



This constant association in the instances cited of contact porphyry 

 copper ores with synclinal or trough structure can scarcely be 

 fortuitous. If not fortuitous it is the expression of a law governing 

 ore deposition at the contact of intrusive masses. What that law is, 

 students of economic geology will do well to ponder before dogmatizing 

 too positively upon the sole efficacy of magmatic waters. 



EFFECT OF INTRUSION ON GROUND-WATER 



Another general circumstance to which the attention of the advocates 

 of the hypothesis of magmatic waters is directed is that they ignore 

 the ground-water. Now no scientific hypothesis is satisfactory that 

 fails to take account of important facts bearing upon the problem in 

 hand. How does Mr. Lindgren and his school dispose of the ground- 

 water? Do they deny its existence? Professor J. F. Kemp seems 

 inclined to deny its existence or at least greatly to minimize its 

 amount below very moderate depths. 43 But his argument is not 

 convincing. Many of the facts recited in his paper tend to prove 

 the opposite of his general contention ; and the apparent dryness of 

 shales, granites or other rocks of minute pore space has but little 



40 Emmons, S. F., Cananea Mining District of Sonora, Mexico, Economic 

 Geology, vol. 5, no. 4, 1910. 



« Lee, M. L., A geological study of the Eliza Mine, Sonora, Mexico, Economic 

 Geology, vol. 7, no. 4, 1912. 



« Op. cit. 



« Kemp, James F., The ground waters, Trans. A. I. M. E., February, 1913. 



