SOME P RE-CAM BRIAN LITERATURE OF NORTH AMERICA 271 



may form instead of the silicate, while in the presence of abimdant 

 oxygen, the development of alternating bands of ferric oxide and 

 silica may take place. Thus by simple laboratory reactions, the 

 composition, textures, and structures of the Lake Superior iron 

 formations are duplicated. 



The monograph cites a number of cases of the intimate associa- 

 tion of sedimentary iron formations of various ages with eruptive 

 rocks in places outside of the Lake Superior region, but fails to name 

 one of the most striking cases which presents a close parallel to 

 that of the Lake Superior region, namely, the oolitic, banded, 

 marine Devonian hematites of the Harz Mountains, which are 

 intimately associated with subaqueous basalt extrusions, diabase, 

 tuffs, and limestones. 



Wright^ discusses the relations of the ophites, oligoclase gabbro 

 and aplite of Mount Bohemia at Lac La Belle on Keweenaw Point. 



The oligoclase gabbro is intrusive into the ophite flows, but the 

 apHte inclosed by the gabbro is regarded as a differentiate from the 

 gabbro. The process of differentiation is supposed to have been 

 accomplished by fractional crystallization, convection currents, 

 and general upward movement of the lava. The intrusives contain 

 veins of sulphides and arsenides of copper which are regarded as 

 genetically related to the native copper deposits in higher horizons 

 of the ophites. The contact metamorphism of the ophites by the 

 gabbro is slight. Diopside has changed to uralitic hornblende, and 

 there is a slight increase in silica and oxidation of iron at the contact. 



' F. E. Wright, "The Intrusive Rocks of Mount Bohemia," part of Ann. Rept. for 

 1909, Michigan Geological Survey, 32 pp. 



[To be continued] 



