16 



BULLETIN OF THE 



various compounds of feldspar and hornblende, forming greenstone or 

 dolorite; or where silica abounds, forming syenite; or serpentine where 



magnesia is in excess It may be stated as a general rule, that 



the great iron deposits of the district occur in close proximity to the 

 igneous rocks, mainly greenstone. This roclc forms nearly all of the 

 prominent peaks of the region, not in continuous ranges, but in a suc- 

 cession of dome-shaped knobs, while the iron ores repose upon their 

 sides, or dip beneath their bases, so that the greenstone appears rather 

 in the form of intercalated beds than as wedge-shaped masses. The 

 ^vhole region has been subjected to a powerful denudation, and the 

 greenstone, being the more unyielding rock, has been left in the form 

 of knobs, or of ill-defined ridges. I cannot recall an instance where it 

 forms a true axis of elevation." {L c, p. 9.) The limonitic ores (soft 

 hematites of the miners) are regarded by him and by Dr. Kimball as 

 formed by the decomposition of the hematite ores in situ {I. c, pp. 24, 

 81). Dr. Kimball also states in this report (p. 87): " Kegarding the 

 bodies of specular iron ores and earthy red luxmatites of the Marquette 

 Kegion as of combined aqueous and metamorphic origin; and, condi- 

 tional to this view, apprehending the stratigraphic arrangement of the 

 general system of aluminous and maguesian silicated schists, among 

 which these beds are intercalated, generally regular and constant as it 

 is, the topography of the country affords tangible data for tracing the 

 hidden conditions of ore beds, and their relation to outcropping rocks. 

 The region is traversed by a series of folds, or undulations, of the entu-e 

 series of rocks, which impart to the surflicc its contour, modified only 

 through subsequent agencies of denudation. Thus the crests of the 

 undulations {i. c, the ridges and hills), originally overlaid by beds of 

 iron ore, which in its purer conditions readily yielded to the abrasion 

 of glacial action, were worn down and commoidy stripped to the under- 

 lying rock." 



In '^ Coal, Iron, and Oil, or the Practical American Miner,'* etc., by 

 S. H. Daddow and Benjamin Bannan, published in 18GG, a decidedly 

 original view of the origin of the iron ores and their associated rocks, as 

 well as of the placer gold of California, is given (pp. 532, 533, 546-550). 

 All are thought to have been formed by volcanic action. "We are 

 aware that all our sedimentary rocks were formed in water, and that 

 the materials forming them are the results of volcanic action. The 

 logical sequence is, that those volcanoes cither existed in water, or 

 vented their lava uito it. Metals are always heavier than their matrix, 

 or the carthv strata in which they are found ; thus, if the lava con^ 



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