The tourmalinic silver-lead type of ore deposit

It is the purpose of this paper to describe a newly recognized type of ore deposit, namely, the tourmalinic silver-lead type.Ore deposits of this kind have been important producers in the Helena mining region, in western Montana, having yielded $5o,ooo,ooo or more.GF•Or.OGtC ENWRON•ma?. •' General oCtatement.-•Thedominant geologic feature of the Helena mining region is the large granite mass which forms the northern extension of the great intrusion .knownas the Boulder batholith.This body of granitic rock was intruded, so far as is now known, in post-Jurassic and pre-Oligocene time, and is in all probability of late Cretaceous age.The cover under which the granite cooled was largely stripped off during the prolonged erosion of the Tertiary period and the granite laid bare, so that now it forms the prevailing surface rock o.f the region.As a consequence of the granite invasion a series of ore deposits, .theprincipal source of the metallic wealth of the •