256 abstracts: geology 



be held fixed for an indefinite time and they are, therefore, termed 

 ''standard." 



It was found to be necessary to apply a correction to the field levels 

 (the levels as actually run) to account for the non-parallelism of equi- 

 potential surfaces. In leveling the correction is an appreciable one 

 when the distance between the surfaces is large. 



The elevations are given in both feet and meters. It was thought 

 best to abandon the custom previously followed (that is, to give the 

 elevations in meters only) , as all topographic maps in this country use 

 the foot as the unit of elevation in contouring. It is also true that 

 all railroads and other engineering organizations use feet entirely for 

 elevations. 



In the future any new lines of precise leveling will be adjusted to 

 fit into the net without in any way disturbing previously adopted ele- 

 vations. As each new line added to the net may have some influence 

 on all or nearly all the elevations, adjustments will be made from time 

 to time which will give the theoretically best values of the junction 

 points of the net, which will be of great scientific interest and value, 

 but such adjustments will not interfere with the standard elevations. 



W. B. 



GEOLOGY. — Geology of the pitchblende ores of Colorado. Edson S. Bas- 

 TiN. U. S. Geological Survey Professional Paper 90-A. Pp. 5, 

 with 2 plates. 1914. 



This account of the mode of occurrence of pitchblende at Quartz Hill 

 in Gilpin County, Colorado, is pubhshed in advance of a much larger 

 report on the same region in which many other types of ore deposits will 

 be considered. The prevailing rocks are pre-Cambrian igneous and 

 sedimentary rocks and tertiary intrusives mainly of monzonitic composi- 

 tion. The pitchblende occurs as a constituent of a number of sulphide 

 veins which traverse both the pre-Cambrian and the Tertiary rocks. 



On the basis of mineral composition the sulphide veins of this region 

 may be divided into (1) the pyritic type and (2) the lead-zinc type. 

 The principal primary minerals of the pyritic type are pyrite and quartz ; 

 chalcopyrite and tetrahedrite are almost invariably present in subor- 

 dinate amounts, and enargite, fluorite, and rhodochrosite occur here and 

 there. The primary minerals commonly present in the veins of the lead- 

 zinc type are galena, sphalerite, pyrite, chalcopyrite, quartz, and calcite. 

 Some parts of the district are characterized solely by one or the other of 

 these t>T5es of mineralization, but in many of the veins both types are 

 present. In such veins it has been demonstrated by repeated exposures 



