abstracts: geology 163 



GEOLOGY.- — Geology and ore-deposits of the Phillipsburg quadrangle, 

 Montana. W. H. Emmons and F. C. Calkins. U. S. Geological 

 Survey Professional Paper 78. Pp. 271, with maps, views, and 

 sections. 1913. 



The consolidated sedimentary rocks range in age from Algonldan to 

 late Cretaceous. The Algonkian Belt series is represented by about 

 20,000 feet of shale, sandstone and impure limestone. It is overlain, 

 with an unconformity that is locally conspicuous, by the Flathead 

 quartzite of Cambrian age, which constitutes the base of a Palaeozoic 

 series about 4500 feet thick, which consists mainly of limestones, but 

 whose uppermost .strata, are quartzitic. These rocks were covered b}^ 

 several thousand feet of Jurassic and Cretaceous sandstone and shale, 

 the latest that survive being of Colorado or Montana age. No struc- 

 tural unconformities are apparent in the Palaeozoic and Mesozoic series, 

 but several epochs are unrepresented by deposits. 



Intrusive igneous rocks occupy a large proportion of the surface. With 

 the exception of a few diabasic and dioritic sills, the important intrusive 

 masses are of irregular or dome-like form and are probably of early 

 Tertiary age. Most of them belong to the granite and diorite families. 

 The Phillipsburg batholith, one of the most important economically, is 

 petrographically similar to the Boulder batholith, but less alkalic. The 

 most unusual intrusives are pj^roxene aplites, associated with grano- 

 diorites or quartz monzonites, some of. which contain primary scapolite. 



The rocks already described are overlain with marked unconformity 

 by Tertiary tuffs and lavas in small amount, and by stream and glacial 

 deposits of Tertiary and Quaternary age. There is clear evidence of 

 two widely separated glaciations. Remnants of Tertiary erosion sur- 

 faces of low relief are well preserved, and the glacial sculpture of the 

 high mountains is striking. 



The structure of the pre-Tertiary sedimentary rocks is highly com- 

 plex. The most remarkable structural features are overthrusts which 

 have pushed Algonkian rocks several miles eastward over rocks as late 

 as Jurassic and which have been folded. These overthrusts are thought 

 to have a close relation with the similar ones in the Rocky Mountains 

 further north and further south, described by Willis, Richards and others. 

 The igneous rocks and the Tertiary gravel deposits have suffered some 

 deformation, but much less than the older strata. 



The ore deposits are classified accorcUng to mode of occurrence as 

 follows: A. Deposits filling fissures. (1) Silver-bearing veins in granite, 

 (2) Gold-bearing veins in granite, (3) Silver-bearing veins in quartzite, 



