SOME PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 581 



southward a distance of about twelve miles. The latter is mostly 

 white and in places coarsely conglomeratic, showing both bedding 

 and cross-bedding. 



Lindgren, Groton, and Gordon^ find small areas of pre-Cambrian 

 rocks, most of them in the northern part of New Mexico, exposed in 

 eroded regions of doming and faulting, flanked by younger rocks. 

 The largest, about 120 miles long, extends as a southward con- 

 tinuation of the Sangre de Cristo range of Colorado, to about 20 

 miles south of Santa Fe. It consists of a sedimentary series, 

 chiefly quartzitic, which with certain greenstone tuffs, amphibohtes, 

 and rhyolites has been invaded by powerful intrusions of normal, 

 usually reddish microcline granite, locally schistose. The granite 

 in turn has been intruded by aphte, abundant pegmatite, and some 

 masses or dikes of greenstone. The sediments are believed to 

 correspond to those of Colorado similarly imbedded in red granite 

 and to the quartzitic schists of the Pinal formation of Arizona, but 

 the pre-Cambrian sediments of the Grand Canyon are believed to be 

 younger than the red granite intrusions. 



Unimportant ore deposits of gold and silver, some zinc and 

 copper, but little lead are found in the granites, gneisses, and 

 amphibolitic schists. Some rich gold placers have developed from 

 them. The ores occur in quartz-filled fissures, usually of the 

 "lenticular" type, shear zones filled with quartz stringers, and 

 disseminations of sulphides in amphibolitic schists. The deposits 

 frequently show evidence of deep-seated origin, such as the develop- 

 ment of heavy minerals, and massive, glassy vein quartz. Many of 

 them were folded and fractured. The ore deposition followed 

 granitic and other intrusions, and the metals were probably 

 delivered from the magmas. 



Noble^ describes the pre-Cambrian succession of the Shimuno 

 area of the Grand Canyon. The following is a summarized 

 section : 



'W. Lindgren, L. C. Groton, and C. H. Gordon, "The Ore Deposits of New 

 Mexico," Prof. Paper 68, U.S. Geological Survey, 1910, pp. — , several maps. 



*L. F. Noble, "Contributions to the Geology of the Grand Canyon, Arizona. 

 The Geology of the Shimuno Area, Am. Jour. Science, 4th Series, XXIX, 369-86. 



