abstracts: geology 363 



lower grade, varying from subbituminous to bituminous coal, in the 

 Paonia shale. In the West Elk Mountains the coals of both members 

 have been changed to anthracite, presumably by the rock movements 

 and the heat attending the intrusion of great masses of igneous rock. 

 Some of the laccoliths were intruded below the coal beds, others within 

 the coal-bearing rocks, and still others above them However, it is a 

 fact worthy of careful consideration in connection with the metamor- 

 phism of coal, that some of the unmetamorphosed coals lie close to the 

 igneous rock and that much of the anthracite lies at considerable dis- 

 tances from any of the known intrusive masses. W. T. Lee. 



GEOLOGY.^ — Geology and ore deposits of the Butte district, Montana. 

 Walter Harvey Weed. U. S. Geological Survey Professional 

 Paper 74. Pp. 262, maps, and sections. 1912. 



A central mass of granitic rock is exposed in the Butte region for 

 about 64 miles in length and 12 to 16 miles in width and is surrounded 

 by upturned and folded sedimentary rocks whose edges are abruptly cut 

 off by the granite. Remnants of a former capping of dark-colored, baked 

 andesites occur as patches over the northern part of the granite region, 

 and extensive areas of these same rocks form mountain masses flanking 

 the granite on the east, north, and west, concealing the sedimentary 

 contact in these areas. These andesitic rocks are partly intrusive but 

 are mainly extrusive lavas or volcanic debris. The granite itself con- 

 tains numerous patches and dikes of white aplite, in places tourmaline- 

 bearing, that are regarded as a later siliceous phase of the granite itself. 

 All these earher rocks, including the sedimentary limestones, etc., are 

 cut by rhyolitic dikes and capped by extensive accumulations of frag- 

 mental rhyolitic rocks and lava flows. Mineral vein formation preceded 

 this later volcanic period and is still in progress in some parts of the 

 district. Over the northeastern part of the granite region quartz veins, 

 usually barren, are extremely common and of unusual size, forming the 

 crests of long, high ridges and dominating the local topography. The 

 deeper older valleys of the district are filled by alluvial and lake beds 

 largely composed of rhyolitic volcano dust and material washed down 

 the slopes when the rhyolitic eruptions occurred. 



The metalliferous ores of the Butte district are of two distinct classes, 

 copper and siliceous silver. The copper ores contain a little silver; the 

 silver ores rarely contain copper; both copper and silver ores contain 

 little gold and the high-grade silver ores contain it in important amounts. 

 The typical ores of the two classes are distinctive and are characterized 



