184 abstracts: economic geology and hydrology 



ECONOMIC GEOLOGY. — Reconnaissance of the ore deposits of north- 

 ern Yuma County, Arizona. Howland Bancroft. Bulletin U. S. 

 Geological Survey No. 451. Pp. 130, with maps, sections, and 

 views. 1911. 



The oldest rocks are granites and gneisses, probably Archaean, together 

 with a group of highly altered sediments, with some igeneous rocks, 

 cut by numerous intrusives of pre-Cambrian age. Some massive 

 granite is provisionally assigned to the Mesozoic, and a series of volcanics 

 with some sediments is regarded as Tertiary. Quaternary deposits 

 are represented by conglomerate and basalt. 



Mineralization took place in the pre-Cambrian, Mesozoic, and Ter- 

 tiary periods. 



Auriferous quartz veins and mineralized shear zones occur in the pre- 

 Cambrian rocks. Another type of auriferous deposit, also of pre-Cam- 

 brian age, is represented by quartz siderite stringers in amphibolite 

 schists and limestones. The largest deposits of copper and iron occur 

 as irregular, veins in shear zones in the amphibolitic and chloritic rocks 

 and as replacements of limestone. Copper also occurs in shear zones 

 and fissure veins in the Archaean gneiss and as fissure veins in the Ter- 

 tiary lavas. Contact-metamorphic copper deposits are found in the 

 zones of metamorphism between the limestones and Mesozoic intrusives. 



A. H. Brooks. 



HYDROLOGY. — The quality of the surface looters of Illinois. W. D. 

 Collins. Water-Supply Paper U. S. Geological Survey, No. 239. 

 Pp. 94, 3 plates. 1910. 



A cooperative investigation was conducted in 1906 and 1907 by the 

 United States Geological Survey, the State Water Survey of Illinois, 

 the engineering experiment station of the University of Illinois, and the 

 State Geological Survey of Illinois for the study of the chemical compo- 

 sition of Illinois waters, their action on boilers, the purification of them 

 for industrial and domestic use, and other similar problems. Stations 

 were established on the principal rivers and reservoirs of the State at 

 26 places where daily samples of water were collected for one year for 

 mineral analysis. The report discusses these analyses in relation to 

 the sources of the mineral matter, its effect in economic uses of the water, 

 and the cost methods of reducing its quantity. A noteworthy feature 

 is the correlation of mineral content with stream discharge, typified by 

 the computations of the relative components of Mississippi River at 

 Chester. The physical and economic conditions that affect the quality 

 of the waters are considered especially in reference to trades wastes. 



