162 abstracts: geology 



into four main groups: Basaltic effusives, granitic intrusives, early 

 intrusives and effusives, and greenstones. P. S. S. 



GEOLOGY. — Iron-ore deposits of the Eagle Mountams, California. 

 E. C. Harder. U. S. Geological Survey Bulletin 503. Pp. 81, 

 with maps, views and sections. 1913. 



The Eagle Mountain iron ores are located in the northern part of 

 the Eagle Mountains, Riverside County, California. The broad 

 structural feature of the northern third of the Eagle Mountains — 

 that is, of the portion consisting of sediments and intrusive granite 

 and where the iron ores occur — ^is an oval dome, extending in a gen- 

 eral east-west direction across the range. The iron ores with associated 

 metamorphic minerals occur as replacements in dolomite. Locally, 

 as at the east end of the area, they have replaced the entire series of 

 dolomite lenses, leaving the associated quartzite but little altered. 

 Elsewhere they occur within the dolomite lenses or beds in bands or as 

 irregular masses roughly parallel to the bedding. The ore is predomi- 

 nantly hematite, but here and there consists of masses of magnetite. 

 Much of the hematite contains disseminated magnetite, making it 

 slightly magnetic, but probably less than 10 per cent of the entire quan- 

 tity of ore is magnetite. 



The succession of geologic events in Eagle Mountain region is as 

 follows: (1) Deposition of sandstone, siliceous shale, and dolomite. 

 (2) Intrusion of porphyritic granite into the sediments. (3) Alteration 

 of granite porphyry to augen gneiss and the sediments to schists and 

 crystalline limestone. (4) Erosion interval followed by submergence 

 and deposition of a great thickness of quartz sandstone; then the depo- 

 sition of arkosic sandstone, followed by the formation of beds and lenses 

 of dolomite and quartz sandstone, and, lastly, of beds of sandstone and 

 conglomerate. (5) Intrusion of quartz monzonite in two main sills. 

 (6) The heat and pressure accompanjdng the intrusion recrystallized 

 and consolidated the sediments and perhaps locally developed meta- 

 morphic minerals. (7) During the latter part of the intrusion, or 

 shortly after it, iron ores and metamorphic minerals were introduced 

 by deep-seated solutions replacing the dolomite and to a slight extent 

 the quartzite. (8) Doming of the sediments and intrusives, accom- 

 panied by great faulting. (9) Erosion exposing all the rock formations, 

 accompanied by the sculpturing of mountains and followed by the de- 

 velopment of great outwash aprons around the mountains. 



A. H. Brooks. 



