116 TUE MARQUETTE IRON-BEARING DISTRICT. 



and the serpentine of the district are regarded "as one group, the serpentin- 

 ous condition prevaihng whenever, locally, greater alteration has taken place 

 in the original rock, which was an eruptive, basic one. It overflowed and 

 mingled with the rocks of the iron-bearing series unconformably, and wliere 

 it is now in contact with them it constitutes, at least in some places, the 

 'soapstone' and the 'chloritic rock' of the mines. Subsequentlv the rocks 

 •of the Huronian were deposited unconformably on the iron-bearing and 

 serpentine groups" (p. 48). The iron-bearing beds are thus placed under 

 the Hur(_)nian, with the unconformity between this series and the Archean 

 above the iron formation, rather than some distance below it, where Irving 

 placed it. 



The geology of the Michigamrae mine is also described, and, north of 

 it, an unconformable contact between granite and quartzite. At this contact 

 is a conglomerate, but the author is not sure which of the two rocks is the 

 overlying one. 



At Marquette the green schists north of the town, which Ir\"ing placed 

 below the Huronian, were examined. These are I'egarded as inseparable, 

 genetically and geographically, from the serpentine group. They are 

 believed to be eruptive, and to underlie unconformably the cpiartzite, and 

 thus to constitute a part of the "iron formation." 



This greeuisb schist * * * is heavy, dark withm, free almost, or entirely, from 

 original ([uartz. On its weathered surface its structure is indicated hy bands of 

 varying shades of green, and in the seams it glitters with hydro-mica. These bands 

 of color are not continuous, but consist rather of an interrupted, narrow striping in 

 which the color lines become lost by running to needle-shaped points or by fading into 

 ■each other. The single light-colored lines sometimes continue for only a few feet or a 

 few inches; and in some cases, when narrow, they rise and disappear in the space of 

 less than an inch, bringing the darker lines into contact so as to present the asi)ect 

 of a nearly homogeneous green rock. * * * In other places * * » the striping, 

 which resembles that of sedimentation, is more evident and persistent, and in some 

 parts coTild be more correctly denominated a banding, some of the bands being 2 or 3 

 inches in width ; but even then they lose their identity in 10 or 12 feet and give place 

 to a finer schisto- fibrous lining. (Pp. 51-52.) 



A plat of Light-House Point is given. Here the schist is made out to 

 be the oldest rock. This is cut by quartz-porphyry and other dikes. 



