494 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



the volcanic focus, while those more remote from it did not altogether lose 

 their Bedimentary structure, hut still hecame altered, and frequently streams 

 of the lower melted or emolliated plastic masses hroke through them, filling 

 transverse ruptures or entering between the ledges parallel with the bedding." 

 (/. c.,p. 23.) 



The rocks thus united Dr. Romingor clanscd as the "Dioritic Group." 

 He showed, as Dr. Wadsworth had already done, the incorrectness of 

 Major Brooks's work, aud how little reliance could be placed on his con- 

 clusions. 



Dr. Rominger appears to have founa in the Menominee iron region a 



condition of things very similar to that existing in the Marquette dis- 

 trict, and arrived at similar conclusions. (/. c, pp. 240, 241.) His idea 

 that the Marquette and Menominee schists arc Huronian means nothing. 

 beyond this, that they appear to him to be hthologically similar to the 

 rocks called Huronian in Canada ; while — so far as his actual work 

 goes — he reaches conclusions regarding the relation of the granitic and 

 schistose rocks identical with those advocated by Foster and Whitney 

 thirty years before. The result of Romingcr's work is decidedly opposed 

 to the division of the Michigan Azoic into two or more formations. 



The question of the origin of the iron ores was not only discussed in 

 the previous portion of this Bulletin, but also elsewhere. (Proc. Bost. 

 Soc. Nat. Hist., 1880, XX., pp. 470-479:) The following indorsement 

 of these views, with-permission to publish it, was received from Mr. A, 

 R. C. Selwyn : 



" I believe with you that many of our great Archean iron-ore beds arc of 

 eruptive origin, while othei-s are stratified iron-sandstones, analogous to those 

 which are now forming along the northern coaats of the St. Lawrence Gulf by 

 the combined action of the rivers and the waves on the more ancient (probably 

 eruptive) ore beds. But such magnetic ores always contain from 40 to 50 per 

 cent of insoluble matter (prol)ably chiefly silica), a much larger proportion 

 than those which are presumably of eruptive origin." 



Professor Dana, in opposition to the view of the eruptive origin of the 

 iron ore and jaspilite, claims that conformability is the evidence used 

 principally in deciding that the ores and schists are alike in their mode 

 of origin. Ho also attempts to decide the point by appeals to other 

 regions. It seems to us that the origin in each region is to be proved 

 by the study of. the district itself, and that arguments from analogy 

 when used alone are fatally defective. 



The question of conformability was the one examined, and upon 

 which the decision regarding the origin of the iron ore was made. A 



