280 Reviews — Ore-deposits of Boulder Batholith, Montana, 



summaries of the history of the various branches of geology and 

 mineralogy in the last hundred years, mainly from the American 

 standpoint, but still with complete acknowledgment of the value of 

 work done in other countries. Of special interest, because some- 

 what novel, is the chapter entitled " A Century of Government 

 Geological Surveys" by Director George Otis Smith, giving a 

 detailed account of the beginnings and subsequent progress of that 

 vast organization known as the United States Geological Survey, 

 which has accomplished such wonders in the exploration and 

 development of the resources of the country. This book leaves on 

 the mind of the reader a feeling of admiration at the many- 

 sidedness, activity, and originality of geology and the allied sciences 

 in America at the present time. 



VIII. — Ore-deposits of the Boulder Batholith of Montana t 

 a Genetic Description. By P. Billingsley and J. A. Grimes. 

 Trans. Amer. Inst. Min. Eng., vol. lviii, pp. 284-361, 1918. 



AT the present time the State of Montana is the most important 

 producer of copper in the world, and the geological relations of 

 its deposits are naturally of the utmost interest. In this paper the 

 authors show in the clearest possible manner the intimate relation 

 that exists between orogenic movements, igneous activity and 

 mineralization. The main folding of the Rocky Mountains, in the 

 Middle Cretaceous, was followed by andesite eruptions in the Upper 

 Cretaceous; and this again by extensive thrust-faulting. The 

 intrusion of the main batholith probably took place in the Eocene, 

 while normal faulting, accompanied by rhyolite flows, extended 

 from the Oligocene to the Pliocene. The intrusion of the batholith, 

 here called the granite stage, is divided into four phases : (1) basic 

 diorite and pyroxenite, (2) quartz-monzonite (the main mass), (3) 

 aplites and pegmatites with tourmaline and other pneumatolytic 

 minerals, (4) quartz porphyry with metalliferous quartz-veins. The 

 mineralization belonging to the monzonite phase is mainly associated 

 with outlying bosses or cupolas rather than with the main intrusion: 

 it includes auriferous pyrrhotite, pyrite and magnetite, as 

 disseminations and contact deposits, the latter also bearing copper 

 and bismuth: the fissure veins also contain copper, zinc and lead: 

 in many localities secondary enrichment is important. The veins of 

 the aplite and pegmatite, phase carry galena, blende, chalcopyrite, 

 pyrite, arsenopyrite and rhodochrosite. The final quartz-porphyry 

 phase gave rise to very rich copper deposits with enargite, tennantite, 

 tetrahedrite, bornite, covellite and chalcocite; these are mostly 

 rather deep within the batholith. 



The earlier andesite stage gave rise to deposits of native copper, 

 magnetite and galena with gold and silver, while the later rhyolite 

 flows are accompanied by gold veins with little sulphide. It is 

 shown that the greater part of the mineralization, perhaps as much 

 as 99 per cent, belongs to intrusions and especially to their later 

 phases : it is therefore magmatic and the richness of the later small 

 phases indicates differentiation and concentration in the magma- 

 basins. There is a definite time-sequence, as follows: (1) contact 



