SOME PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 263 



Crane^ describes the pre-Cambrian iron ores of Iron Mountain, 

 Missouri, as veins of specular hematite containing apatite and 

 tremoHte, which occur in porphyry; and detrital ores derived from 

 the vein ores. The ores of Shepherd Mountain near by are also vein 

 ores, but their principal constituent is non-titaniferous magnetite, 

 with pyrite and a little clay. The Pilot Knob ores are soft blue, 

 banded hematites, locally with ripple marks, and rest conformably 

 on a nearly smooth floor of porphyry, and grade laterally into a 

 porphyry breccia. The formation is separated into two parts by a 

 clay like bed. The ore above the clay like bed is more distinctly 

 stratified and grades upward into a hundred feet of breccia. 



Crane concludes that these ores represent replaced tuffs, from 

 the fact that they grade into breccias both along the strike and the 

 dip. It has been found extremely difficult to explain the origin of 

 the Pilot Knob ores under any hypothesis offered thus far. This is 

 due in large part to the fact that only a small remnant of the ores, 

 and the rocks associated with them have been left by erosion. The 

 principal objection to the replacement hypothesis is found in the 

 interstratification of non-iron-bearing materials with the ores. 

 Under the replacement hypothesis it would be necessary to assume 

 that the replacement of certain layers was accomplished, while 

 others remained unaltered. 



Crane does not correlate the pre-Cambrian rocks of Missouri 

 with certain Lake Superior pre-Cambrian rocks, as some of his 

 predecessors have done. 



Grout^ infers from analyses of Keweenawan diabase in different 

 stages of alteration that the fresh rocks contain more copper than 

 the altered, the copper being in the form of a basic silicate in the 

 fresh rock, and that the more basic diabases originally contain more 

 copper than the more acid types. 



Grout^ finds that chemical and mineralogical analyses of the 

 Keweenawan lavas of the Kettle River region of Minnesota indicate 



' G. W. Crane, "Iron Ores," Missouri Bur.. Geol. and Mines, Vol. X, (191 2) pp. 

 36-39, 107-44. 



= Frank F. Grout, "Keweenawan Copper Deposits," V, No. 5 (1910), 471-76. 



3 F. F. Grout, " Contribution to the Petrography of the Keweenawan. Joiir. Geol. , 

 XVIII, No. 7 (1910), 633-57. 



