abstracts: geology 23 



GEOLOGY. — Mining districts of the Dillon quadrangle, Montana, and 

 adjacent areas. Alexander N. Winchell. U. S. Geological 

 Survey Bulletin No. 574. Pp. 191. 1914. 



The Dillon quadrangle lies in southwestern Montana and is in gen- 

 eral mountainous, but it contains one large valley, that of Beaver- 

 head and Jefferson rivers, and numerous smaller valleys. Sedimentary 

 rocks ranging in age from Algonkian, possibly Archaean, to Quater- 

 nary, are represented in this province. Igneous rocks include quartz 

 monzonite, occurring in a large batholith, together with various other 

 types of intrusives; volcanics also occur. 



The geologic structure was determined largely by the intrusion 

 of the great Boulder batholith, which seems to have penetrated by fault- 

 ing or thrusting, or by assimilation of material, or by updoming impor- 

 tant areas in the quadrangle in Tertiary time, or, more probably by a 

 combination of all these processes. 



The principal mineral resources of this area and those adjacent in- 

 clude gold, silver, copper, and lead, and smaller quantities of zinc, 

 iron, manganese, tungsten, antimony, arsenic, bismuth, vanadium, 

 tellurium, and sulphur. 



The Boulder batholith in a large sense seems to be in whole or in part 

 the source of many of the ore deposits of the Dillon quadrangle. The 

 ore deposits occur chiefly in fissure veins or in irregular bodies pro- 

 duced by contact metamorphism in limestone or dolomite. Some 

 of the ore deposits are in the form of fissure veins. Others belong to 

 the type of ore disseminated in the country rock. 



Alfred H. Brooks. 



GEOLOGY. — Electric activity in ore deposits. Roger C. Wells. 

 U. S. Geological Survey Bulletin No. 548. Pp. 1-78. 1914. 

 Many of the chemical reactions that occur in ore deposits involve 

 oxidation and reduction, and with electrolytes the phenomena of oxi- 

 dation and reduction are closely linked with electric activity. Many 

 ores are also conductors of electricity. It is therefore possible, and 

 necessary to consider, that balanced chemical action may occur in ore 

 deposits at two points somewhat removed from each other. Such ac- 

 tivity would produce different mineral associations than would result 

 from a direct admixture of the chemical agents. One of the simplest 

 possible combinations by which electric action could occur would con- 

 sist in the presence of two different active solutions in contact with a 

 single body of ore, the two active solutions being united by any "in- 



