192 SEVENTH REPORT. 



The felsite has a fine grained chocolate bro\^^l ground mass, and carries phenocrysts of 

 orthoclase and quartz. 



4. Interbedded sedimentaries and eruptives: The thickness is placed at 5,400 feet, but 

 it may be greater, since a part or even the whole of, the space without outcrop may be un- 

 derlain by these rocks. If so this division would extend downward about 1,000 feet more 

 than is here indicated. 



This division contains sedimentaries and basic eruptives. The former have so increased 

 in ijroportion to the latter that one cannot help but notice the difference from the great 

 thickness of eruptive rocks below the Chippewa felsite. Below the felsite the per cent of 

 sedimentaries is practically nothing, but from it up to the top of the last eruptives they 

 form about 25 per cent of the rock. The per cent of sedimentaries increase as we go up- 

 ward in the division itself. 



Some of the eruptive beds contain five or six flows, and some less. They are, for the 

 most part, ophites. The lava flows are all thin, mostly under 150 feet, and the average 

 thickness would not exceed 100 feet. All have amygdaloidal surfaces. The weathering 

 seems to be much greater in these flows than in those below the Chippewa felsite. The 

 amygdules are largely calcite, chlorite and laumontite. 



There are at least six sedimentary beds in this division, and all are composed of acid 

 material Sometimes these beds are purely sandstone, and sometimes conglomerate. 

 Others shew both phases. There is a great variety of pebbles but the felsite pebbles 

 form nearly the whole mass. The highest sedunentary bed of this division is a conglom- 

 erate 400 feet thick. The highest eruptive bed is 350 feet thick, and marks the close of 

 the eruptive action of Keweenawan times. The top of this eruptive is probably the toiD 

 of Irving's Lower Keweenawan. 



5. Sedimentaries only: The highest division that is made in this paper contains sedi- 

 mentaries only. The rock at the base of the division is a conglomerate. Some of the 

 boulders are nearly a foot and a half in diameter, and from these there are all sizes do'mi 

 to the smallest sandstone grains. All the pebbles are water-worn and well rounded. There 

 are pebbles of amygdaloid, granite, diorite, syenite, quartzite, jasper, labradorite-por- 

 phyrite, and felsite-porphyrite, but all these are few in number and it is putting it quite 

 strong enough to merely say that they are present, for nearly the whole mass is made up of 

 different varieties of felsite pebbles and boulers. The finer material between the pebbles 

 is also acid. As we go upward we find that the conglomerate gradually changes to a sand- 

 stone, and the last 300 feet shows no conglomerate at all except a thin band of 10 feet 

 about' 50 feet from the top. The sandstone has a red color, but in some places it is cjuite 

 brown. All through it there are seams of white sandstone varying in thickness from a 

 quarter of an inch up to one foot. It has a marked tendency to split along its bedding 

 planes, cleaving almost like shale, and for this reason will never be of value as a building 

 stone. 



Above this thick conglomerate bed is the Nonesuch formation. Only the lower contact 

 is exposed on the Black River and that at the very mouth. About five miles westward 

 on the lake shore the upper contact is exposed, and the shale is seen to have a thickness 

 of not more than 300 feet. This formation displays the typical greenish colored Nonesuch 

 shales interbedded with Nonesuch sandstone. These are the youngest Keweenawan rocks 

 exposed on the Black River. 



Tlie rocks described above are all exposed in the Black Eiver section, 

 and the thicknesses given here are calculated from the exposures in that 

 section. Irving saj^s of the Lower Keweeawan in his monograph on the 

 copper-bearing rocks vol. V, U. S. G. S., p. 158, ''On the Montreal river, 

 taking the surface width and dip angles together, the apparent thick- 

 ness is as much as 33,000 to 35,000 feet, but how much of this may be 

 due to the continuation westward of the Keweenaw fault, or whether 

 this fault extends so far as this, it is impossible to say. It certainly 

 does not extend much farther and, from its evident rapid decrease in 

 tlirow from the Ontonagon Kiver westw^ard, it seems that its influence 

 on the Montreal cannot be great." At the base of our section the Mon- 

 treal is about six miles west and at the top it is as much as eighteen 

 miles. The thickness of the Lower Keweenawan on the Black River 

 appears to be as much as 42,000 ft. 



The section that was made on the Black River was small and does not 



