FEB. 4, 1921 ABSTlLiVCTS: GEOLOGY 57 



of more than 300 feet. They also inhabit distributing pipes and intakes 

 of city water systems where the water contains an appreciable percentage 

 of iron, and at times they become obnoxious in such situations. 



The best-known iron-depositing bacteria such as Crenothrix, Leptothrix, 

 and Spirophyllum belong to the group of organisms known as thread bac- 

 teria. Many coccus and bacillus forms also are known, which have the 

 power of precipitating ferric hydroxide or basic ferric salts from iron-bearing 

 solutions. Some iron-depositing bacteria require iron compounds in solu- 

 tion for their life processes; others do not require iron compounds, but use 

 them when present. Some require inorganic iron compounds ; others organic 

 iron compounds, and still others use both. Many of these organisms have 

 been cultivated on artificial media. 



The importance of iron-depositing bacteria in the formation of iron- 

 ore deposits is problematical. They are known to form bog-ore deposits 

 and have been found depositing limonite in openings at moderate depths. 

 Most of the large sedimentary iron-ore beds, however, are known to have 

 been deposited in salt or blackish water and iron-depositing bacteria have 

 not been found up to the present in such situations. E. C. H. 



GEOLOGY. — The copper deposits of Ray and Miami, Arizona. F. L. Ran- 

 SOME. U. S. Geol. Survev Prof. Paper 115. Pp. 192, pis. 54, figs. 29. 

 1919. 



The Ray and Miami districts lie about IS miles apart in central Arizona, 

 in the belt of mountain ranges that borders the Arizona Pla eau along its 

 southwestern edge. From the beginning of modern operations in 1907 

 to the end of 1918 these districts have yielded 1,098,409,607 pounds of 

 copper and the three principal copper companies have declared dividends 

 amounting to $67,592,552. 



The rocks of the region comprise pre-Cambrian schist and granite over- 

 lain by Paleozoic sedimentary rocks and limestones which in turn are un- 

 conformably overlain by Cretaceous andesitic rocks. Above these in the 

 stratigraphic column are lavas and fluviatile detrital deposits of Tertiary 

 and Quaternary age. The principal intrusive rocks are diabase, probably 

 of early ^Mesozic age, and granite, granodiorite, quartz diorite, and related 

 porphyries, probably of early or middle Tertiary age. 



The principal copper deposits are of the enriched disseminated type and 

 their most valuable constituent is chalcocite. The tenor of the ore ranges 

 from about 1.5 to 6 per cent and about 260,000,000 tons were estimated as 

 available at the beginning of the present mining operations. The ore bodies 

 are undulating, flat-lying masses of more or less indefinite horizontal out- 

 line and of varying thickness. In a verv'- general wa}' the}' are marginally 

 situated with reference to intrusive masses of granite, granite porphyry, 

 and quartz monzonite porphyry, but the ore occurs both in the pre-Cam- 

 brian schist and in the Tertiar}^ intrusive rocks. By far the greater part 

 of the ore is in schist. The ore bodies are the result of the operation of 

 two general processes — upward or hypogene metallization as a consequence 

 of the intrusion of granite or monzonite porphyries, and downward or super- 

 gene enrichment by percolating atmospheric water. 



Supergene enrichment has generally been treated as a continuously pro- 

 gressive process. There is considerable probability, however, that it is 

 essentially cyclic, although the cyclic character may not be patent in all 

 deposits. The essential fact appears to be that as enrichment progresses 



