318 University of California Publications in Geology [Vol. 8 



fissure. Evidently the material had made room for itself by stoping 

 or similar mechanical process. In the neighborhood of these dykes 

 there are dyke-shaped zones of extremely altered and softened ore- 

 porphyry in which, in many cases, nothing is left save soft ash-like 

 sponge of silica, blackish or grayish, and suggesting attack by hot 

 alkaline solutions which might have ascended from the source of the 

 clastic dykes. The suggestion is therefore ventured that these mud 

 dykes were preceded by gaseous or fluid emanations which softened 

 the rock, rendering it removable by the mechanical stoping effect 

 incident to the pulsating ascent of the material. This is somewhat 

 borne out by the occurrence, in the dykes, of fragments of this same 

 ash-like material. 



In the dykes were found fragments of limestone, monzonite, spongy 

 psilomelane, and secondarily enriched ore-porphyry exactly similar to 

 that now being mined. There are also fragments of rhyolite similar 

 to that ejected in Tertiary time as flows and pyroelastics, which are 

 taken as indicating that this manifestation must have been a waning 

 phase of the eruption of the rhyolite. 



The particular significance of the occurrence is that the fragments 

 came both from the depths and from near the surface, and that they 

 are in many cases rounded as if by abrasion due to pulsations or 

 surging in the ascent of the material, and that the date of the event 

 was subsequent to the chalcocite enrichment of the ore-porphyry. 

 Incidentally it is of interest as another proof that volcanic agglom- 

 erates may form not only at a vent, but also in the depths below it, 

 and be poured out on the surface ready made as a volcanic mud flow. 



CONCLUSION 



Briefly, the conclusions drawn from these observations may be thus 

 recapitulated : first, that the matrix of the ores is an altered monzonite- 

 porphyry, and that this rock is the oldest of the local monzonitic 

 rocks which have intruded the Devonian and Carboniferous sediments, 

 as a series of stocks; second, that the chief agency in producing the 

 present state of alteration of this rock was descending meteoric water, 

 and that the present ores are due essentially to a diffused secondary 

 deposition of chalcocite by this process; third, that this enrichment 

 occurred above the ground water-level; and fourth, that it had 

 occurred before the eruption of the rhyolite. 



Transmitted September 26, 1914. 



