MUSEUM OF COMPAKATIVE ZOOLOGY. 27 



the presence of ei*iiptive rocks, and their accompanying chemical agencies. 

 Foster and "Whitney regarded the " diorites " of this region as eruptive 

 rocks, but Rivot, Kimball, Hunt, Winchell, Credner, Brooks, and Wright, 

 as sedimentary ones and interstratified with the schists. 



The iron ores are regarded as iill of sedimentary origin by Foster, 

 Kimball, Dana, Hunt, Winchell, Credner, Brooks, Newberry, and Wright, 

 but are believed for the most part to be of eruptive origin by Whitney, 

 and by Foster and Whitney. These ores were said to be in the upper 

 portion of the Huronian series -by Kimball, Brooks, and Wright, with the 

 " diorites " underlying them. 



It will thus be seen that, while Foster and Whitney regarded certain 

 of the rocks in the "Huronian" as eruptive, Hubbard, Rivot, Kimball, 

 Hunt, Credner, Brooks, and Wright regarded all, with a few slight excep- 

 tions, as sedimentary ; and Houghton, Hubbard, Locke, Kimball, Rivot, 

 and Brooks teach that they pass by gradual transition into one another. 



The most important points, then, about which there has been or is 

 difference of opinion, are the age and relation of the granite and schists, 

 the origin of the diorites and iron ores, the passage of one rock into 

 anotlier, and the presence or absence of eruptive rocks. These and other 

 questions relating to this district admit in many cases of no middle 

 ground ; one or the other party must be mistaken in their observations 

 or conclusions, or both. All these questions lie closely to the fundamen- 

 tal propositions of geology; they reach to the superstructure of the 

 science. 



Methods of Observation. 



The object of the writer in visiting the Iron district was to clear up 

 some of the preceding mooted points in the geology of that region, if 

 possible, especially the origin of the iron ores and their relations to the 

 country rock. 



From our personal experience in both regions, we should hold that 

 the ordinary methods of geological research which ai'e employed in the 

 study of the comparatively undisturbed and unaltered sedimentary 

 rocks of the Mississippi Valley are not sufficiently accurate for our pur- 

 pose in the Lake Superior district, where the rocks are foliated, dis- 

 turbed, and of mixed eruptive and sedimentary origin. Stratigi-aphical 

 laws that hold good in the foi^mer region do not in the latter, especially 

 when it is sought to connect together two rocks of unlike character, or 

 when the nature and origin of either or both is a point of dispute. In 

 this disturbed district the presence of a rock in one place will hardly 



