84 Scientific Intelligence. 



The rocks immediately about Tonapah are all volcanic in char - 

 acter. They are of Tertiary age as is proven by their relations 

 to the sedimentary rocks of surrounding areas. The earliest lava 

 is an andesite which came probably in the early Miocene and in 

 this rock the fracture zones were formed along which the valu- 

 able veins of the region were deposited. After the formation of 

 the veins a second eruption of an andesite of somewhat different 

 composition took place, following which came another period of 

 quiet and erosion. Eruption was resumed by ejection of sili- 

 ceous dacite and fragmental material. Later a more glassy 

 dacite was poured out on the surface as thin sheets or through 

 explosion deposited as tuffs. ■ Hot waters ascending along the 

 contact of this lava formed quartz veins carrying gold and silver. 

 These veins, however, are relatively unimportant in the district. 

 In a lake, formed subsequent to these last eruptions, several hun- 

 dred feet of sediments were laid down which were later lifted, 

 eroded and with the whole district tilted to the west at an angle 

 of 20°. Then followed another period of volcanic activity, 

 chiefly explosive in type and subsequently the volcanic vents 

 themselves were filled by lava columns. At this period the rocks 

 were broken into blocks and faulted. 



The most important veins occur in the earlier andesite and do 

 not extend into the overlying rocks. They were deposited by 

 hot waters which succeeded the intrusion of the andesite and 

 ascended along zones of fracturing in it. In many cases the vein 

 consists simply of a zone of more or less altered andesite. This 

 zone is cut by parallel fractures having the same strike and dip 

 as the walls, which in turn are nothing more than strong fractures 

 of the same kind. In other cases the action has proceeded fur- 

 ther and a part or at times the whole of the andesite is 

 replaced by quartz. Cross fractures evidently greatly influenced 

 the circulation along the main fissure zones and so in part con- 

 trolled the localization of the rich ore bodies. 



The primary ore minerals are chiefly sulphides of silver, anti- 

 mony, copper, iron, lead and zinc, silver selenide and gold, while 

 the chief gangue minerals are quartz and adularia. Where the 

 veins have been exposed to the oxidizing action of surface waters 

 some alteration of the original minerals has taken place, cerargy- 

 rite becoming plentiful and some secondary silver sulphides being 

 found. w. e. f. 



4. The Lead, Zinc and Fluorspar Deposits of Western Ken- 

 tucky ; by E. O. Uleich and W. S. Tangier Smith. IT. S. G. S. 

 Professional Paper, No. 36, 218 pp., 15 pis., 31 figs. — This paper 

 is a description of the general geology and mineral deposits of 

 an area situated in western Kentucky and extending into the 

 southern portion of Illinois. The region is one of Carboniferous 

 sedimentaries, made up chiefly of limestones with subordinate 

 amounts of sandstone and shale. Structurally these rocks are in 

 the form of two truncated domes but have been extensively 

 broken up and displaced by complicated faulting. Vertical dikes 



