268 EDWARD STEIDTMANN 



in the form of chloride. The precipitation of the copper Lane 

 believes took place according to the experimental results of 

 Fernekes, the copper being taken out of solution by means of ferrous 

 chloride in the presence of alkalies, principally in the form of calcite 

 and prehnite and datolite, since the copper shows most intimate 

 associations with these minerals. 



Paige^ states that the pre-Cambrian rocks of the Llano-Burnett 

 region consist of a sedimentary series of gneisses and schists and 

 banded iron formation lenses composed of alternating layers of 

 magnetite, quartz, and silicates, the whole intruded by a large 

 development of granites and minor basic igneous rocks. 



Richardson^ states that the lowest pre-Cambrian rocks of the 

 El Paso district are white and red quartzites, fine, round grained, 

 intruded by diabase dikes. About i,8oo feet are exposed in the 

 Franklin Mountains. The dip of the beds is 20°-45° west. They 

 are separated by a sKght unconformity from 1,500 feet of massive 

 red rhyolite porphyry overlying them. The rhyoHte is separated 

 by a marked unconformity from the Cambrian beds above it. 



Sharwood^ publishes a large number of analyses of the rocks, 

 minerals, ores, and waters of the Homestake mine, as a supplement 

 to Professional Paper No. 20 of the U.S. Geological Survey, 



Steidtmann'' finds that a minority of the joints of the Baraboo 

 quartzite are clearly related to the local folding; namely, joints 

 parallel to the bedding, and strike joints which dissect the bedding. 

 The abundance of the joints is related to the intensity of the folding. 

 The majority of the joints are vertical, gaping, continuous cracks, 

 which are not related to the local folding, but probably belong to 

 larger units of structure. 



Todd^ states that the pre-Cambrian rocks have been penetrated 

 by borings in the Aberdeen-Redfield quadrangles of South Dakota. 



^ Sidney Paige, "The Mineral Resources of the Llano-Burnett Region, Texas," 

 Bull. 450, U.S. Geological Survey, 191 1, pp. 103, maps and illustrations. 



2 G. B. Richardson, "The El Paso Quadrangle," U.S. Geological Survey, Folio 166, 

 1909, II pp., 2 pis., maps, etc. 



3 W. J. Sharwood, "Analyses of Some Rocks and Minerals from the Homestake 

 Mine, Lead, South Dakota," Econ. Geol., VI, No. 8, pp. 729-89. 



4 "The Secondary Structures of the Baraboo Quartzite," Jotir Geol., XVIII, No. 

 3 (1910), 259-70. 



5 J. E. Todd, "The Aberdeen Redfield Folio," U.S. Geological Survey, Folio 165, 

 1909, 13 PP- 



