MAGNETIC OBSERVATIONS. 339 



The other formation which produces disturbances is that which I have 

 correlated with the Negaunee formation and named in a former paper^ the 

 Michigamme jasper, but which is here renamed the Groveland formation. 

 This rock, while varying a great deal in character, is generally much like 

 that magnetic phase of the Negaunee formation in which the griinerite is 

 rare or absent. From the fact that it now survives for the most part only 

 in shallow and shattered synclines, it often lacks the regular banding; and 

 hematite is always present in greater or less amount. The relative propor- 

 tions of the two iron minerals vary along the strike also. The rock as a 

 whole, however, is very magnetic, but not so strongly so as the Negaunee 

 formation in the Republic trough. 



In the Felch Mountain range there is still a third magnetic formation, 

 which seems to overlie unconformably the lower series, and is therefore 

 provisionally assigned to the Upper Huronian. This formation consists of 

 ferruginous schists, interstratified with layers of ferruginous fragmental 

 quartzite. It is generally much less highly inclined than the magnetic 

 rocks of the lower series as well as less rich in iron, and the disturbances 

 produced by it are correspondingly small. 



Besides these rocks of sedimentary origin, with which this paper prop- 

 erly deals, it may be mentioned that along the Fence River there is a 

 considerable area of metamorphic eruptives, which are often exceedingly 

 magnetic. These are restricted to a definite geological horizon, within 

 which the magnetic disturbances are remarkable for their complexity and 

 irregularity, no doubt as the result of a very irregular distribution of 

 magnetite and of the formations which chiefly contain it. The rocks in 

 portions of this belt outcrop freely, and the disturbances can therefore 

 easily be assigned to the proper causes. 



SECTION III. THE DISTRIBUTIO^T OF MAGNETISM IN THE MAGNETIC 



ROCKS. 



Magnetite occurs, therefore, in these Algonkian rocks in different 

 ways. In some instances it is mainly concentrated in nearly pure parallel 

 layers ; in others, it is more or less evenly disseminated through non- 

 magnetic material; and still again it is present in both forms. Moreover, 



' Relations of the Lower Menominee and Lower Marquette series of Michigan (Preliminary), 

 by H. L. Smyth: Am. .Jour. Sci., Vol. XLVII, 1894, pp. 217, 218, 223. 



