Miscellaneous Intelligence. 333 



tiary time. Immense flows of later Tertiary lavas which lie 

 unconforrnably upon the earlier rocks are found to the north of 

 the district and in some places extend down into this quadrangle. 



No great disturbance of the region occurred until the intru- 

 sions of the porphyries, which caused extensive breaking and dis- 

 location of the sedimentary rocks. Subsequently folding must 

 have taken place, and the uplifted area broke into fragments, 

 which gradually settling down, formed the rocks into monoclinal 

 blocks. 



The ore deposits of the district are always found within or 

 closely adjacent to the porphyry intrusions and are considered to 

 have an intimate genetic relation with these rocks. They occur 

 in two main forms ; either as tabular bodies in strongly meta- 

 morphosed beds of limestone or shale, or as veins in fissures 

 which traverse all of the rock types of the district. The intru- 

 sions of the porphyry produced in the adjoining limestones and 

 shales important contact-metamorphic action which resulted in 

 the metasomatic development of garnet, epidote, diopside and 

 other silicates, accompanied by pyrite, magnetite, chaleopyrite 

 and sphalerite. The sulphides were not later introductions but 

 contemporaneous in their formation with the other contact min- 

 erals. The contact zone received large additions of oxide of 

 iron, silica, sulphur, copper and zinc, which it is believed were 

 given off by the intruded magma and forced through the adjoin- 

 ing sedimentary beds. These deposits when unaltered are every- 

 where of too low grade for profitable extraction, but have in 

 many places been attacked by oxidizing surface waters and 

 greatly enriched in value. Workable ores of this class are almost 

 wholly oxidized, being made up chiefly of the two carbonates, 

 malachite and azurite. 



The vein deposits were formed somewhat later, but probably 

 while the igneous rocks were still hot and giving off mineral- 

 bearing waters which, circulating through the rocks, deposited the 

 ore materials in the fissures, forming normal veins largely of the 

 replacement type. The bulk of these deposits consist of pyrites 

 with a copper content in the ore, usually below one per cent. But 

 descending oxidizing waters have served to enrich these deposits 

 also, forming in them a belt of chalcocite ore varying between 

 200 and 250 feet in thickness. During the earlier days the rich 

 ores from the oxidized deposits in limestone and shale furnished 

 the major part of the output, but at present the camp is depend- 

 ing chiefly on these large low-grade bodies of chalcocite ore. 



w. E. F. 



III. Miscellaneous Scientific Intelligence. 



1. A Contribution to the Oceanography of the Pacific ; by 

 James M. Flint. Bull. U. S. National Museum No. 55, pp. 62, 

 pis. 14, 1905. Compiled from data collected by the United 

 States steamer Nero in 1899 while engaged in the survey of a 

 route for a trans-Pacific cable. — The instructions regarding the 



