﻿R. D. Irving — Ferruginous Schists and Iron Ores. 255 



Hawaii in 1839 and 1840. The idea is now no longer a half- 

 studied conclusion, but, thanks to the elaborate investigations 

 of Messrs. Hague and Iddings, and of Allport, Judd and others, 

 it is an established fact in the science of igneous rocks. * 



Aet. XXIX. — Origin of the Ferruginous Schists and Iron Ores 

 of the Lake Superior region ; by R. D. Irving. 



(Published by permission of the Director of the U. S. Geological Surrey.) 



Two classes of theories have been maintained to account for 

 the iron ores of the Lake Superior region. According to one 

 of these, these ores, with more or less accompanying rock ma- 

 terial, and particularly the jaspery schists so often associated 

 with them, are in the main of eruptive origin ; according to the 

 other they are of sedimentary origin, the original sedimentation 

 having been mechanical, or chemical, or both, according to dif- 

 ferent theories. On the eruption theories subsequent chemical 

 alteration is allowed to come in to some extent to explain 

 present conditions, as it is also with some of the sedimentation 

 theories; but the latter theories appear to appeal in the main 

 to a regional metamorphism as the cause of present conditions. 



Those who have maintained the theories of a sedimentary 

 origin have relied chiefly upon the common intimate interlami- 

 nation of siliceous and ferruginous materials; upon the manifest 

 restriction of the ores and jaspery schists to definite stratigraph- 

 ical horizons ; upon their interfolding with other members of the 

 same series, and upon their apparent gradation in places into 

 plainly fragmental deposits. These conditions being taken to 

 indicate original sedimentation, different authors have imagined 

 the unaltered deposits to have been argillaceous carbonates 

 like those of the coal measures; to have been brown ores like 

 those found under bogs, or accumulating in shallow lakes, at 

 the present day ; or to have been magnetic iron sands like 

 those of modern sea shores. All of these theories appear to re- 

 gard the silica of the jaspery schists and ores as having been 

 sand ; its present non-arenaceous, non-fragmental condition be- 

 ing taken to be the result of metamorphism. 



Advocates of an igneous origin for the iron ores and asso- 

 ciated jaspers have been few ; and it is noteworthy that their 



* My twenty-five days at Tahiti, in the early autumn of 1839, were divided up 

 between the study of the coral reefs, the collection and study of the Crustaceans 

 of the coast (both of which were part of my assigned duty), and geological 

 explorations over the island ; and our day of departure came almost immediately 

 after my ascent of Mt. Aorai. in which I had become aware of the deep interest 

 of the problems the island illustrates. My visit to the Hawaian Islands came a 

 year later. 



