MAGNETITE AND PYRITE. 127 



stone in chlorite slate. A great ore body of magnetite described 

 as a vein has lately been reported from San Bernardino County. 

 It is said to be from 30 to 150 feet thick, and to lie between dolo- 

 mitic limestone and syenite. 1 A great bed of a kind not specified 

 is reported from San Diego County. 2 



2.03.11. Example 13. Cornwall, Penn. Immense beds of 

 soft magnetite, associated with green slates, limestones of Cambrian 

 age, and Triassic sandstones. They are pierced by dikes of Trias- 

 sic diabase. The exact geological relations of these beds have 

 been greatly disputed, the doubtful point being whether they are of 

 Triassic or Siluro-Cambrian age. They occur just at the juncture. 

 of schists of the latter with red sandstones of the former. The 

 most probable explanation is that they were originally an immense 

 deposit of Siluro-Cambrian limonite (Example 2a), which has been 

 changed to magnetite by the diabase eruptions. They have been 



LIMESTONEj 



CORNWALL MINE 



H SECTION ON LINE A.B.OF MAP -t 



| i ii i -i 350 FT. - 



FIG. 28. Cross section of the magnetite ore body at Cornwall, Penn. 

 After Bailey Willis, Tenth Census, Vol. XV., p. 226. 



thought eruptive also, an idea far less probable. The ore forms 

 two low hills. It is mined by open cuts. The amount is enormous, 

 600,000 to 700,000 tons being shipped yearly. Up to 1888 more 

 than 8,500,000 tons had been mined. The ore is rather low in iron 

 but is not high in phosphorus. Considerable amounts of copper 

 ores (chalcopyrite, etc.) occur in fissures crossing the magnetite. 

 Farther southwest, in York County, the Dillsburg group of 

 mines affords specular hematite and magnetite from deposits in 

 Triassic sandstones. 3 



1 Ann. Rep. State Mineralogist, 1889, p 235. 



2 Ibid., 1889, p. 154. J. R. Browne, " Mineral Resources West of the 

 Rocky Mountains," 1868. C. King and J. D. Hague, "Mineral Resources 

 West of the Rocky Mountains," 1874, p. 44. H. G. Hanks and W. Irelan r 

 Ann. Reps, State Mineralogist, California. (Very little on iron.) F. von 

 Richthofen, private reports quoted in Tenth Census, Vol. XV., p. 495, 

 J. D. Whitney, Geol. Survey Gal., Vol. I. 



3 P. A. Fraser, " Study of the Specular and Hematite Ores of Iron of 

 the New Red Sandstone in York County, Pennsylvania," M. E., V. 132. 



