MICHIGAN ACADEMY OF SCIENCE. 191 



exposed and probably marks the top of Irving's Lower Keweenawan. 



Above this point only sedimentaries occnr, and their exposures on the 

 Black Kiver show them to be conglomerates sandstones and shales. 



I shall consider these rocks briefly under the heads that I have out- 

 lined above. There is no real reason why such a division should be made, 

 except for the sake of convenience in this present paper, and yet the 

 division can no doubt be excused since there is represented in each a 

 diflerence in the way in which nature has displayed hereself. 



1. Basal sandstone 200 ft. 



2. Eruptives with little or no sedimentary rock . . . 35400 ft. 



3. Felsite with a known thickness of 400 but with a 



possible 1400 ft. 



4. Interbedded sedimentaries and eruptives 5400 ft. 



5. Sedimentaries 1500 ft. 



Total 43900 ft. 



1. The basal sandstone: I shall not add anything to what I have already said about this 

 sandstone. 



2. Eruptives with little or no sedimentaries: At the base there are amygdaloidal with 

 aniygdules mostly of quartz, many of which are agate. Above them are labradorite por- 

 phyrites with small phenocrysts of feldspar. Not far from the base is a thin bed of very 

 coarsed grained gabbro. Its presence is interesting since it is, I think, the first exposure 

 of the Bad River gabbro noted in the Keweenawan of Michigan. In all probability it is a 

 sill. Above the gaobro there is a series of flows partly melaphyres, and partly labradorite- 

 porphyrites. There are, in all, several flows of these poryphyrites one of which has pheno- 

 crysts of feldspar 30mm. in length. About 7,500 feet from the base of the Keweenawan 

 there is a thin felsite-porphyrite and above it are exposures of ophite. Then follows the 

 space without exposure which is here being considered as containing eruptives only. It is 

 possible, though not probable, that there is considerable sedimentary rock in it. Beyond 

 this drift covered part is a series of lustre mottled melaphyres. Some have mottling as much 

 as an inch across but the most of the niottling is much smaller. These ophites continue 

 ahnost up to the Chippewa felsite. 



This body of eruptives is not wholly without sedimentaries for there are some sand- 

 stones just beneath the felsite and at least one bed farther down with a thickness of 100 

 feet. But it is quite certain that the sedimentaries are small factors. It is interesting to 

 notice here that although the great mass of Keweenawan rocks up to this point are basic, 

 yet what few sedimentaries are present are acid. 



There does not seem to be any flows of very great thickness in all this great body of 

 eruptives. At the very base where the rock is well exposed on the Bessemer bluffs the dif- 

 ferent flows are readily distinguished and they show different thicknesses from 10 up to 

 250 feet. It is quite certain that the larger lustre mottling in 21-48-46 is indicativie of 

 thicker flows, as is also the coarser grain of the rocks on the highlands to the east of Chip- 

 pewa Bluff. But it seems quite evident that there are no very thick flows. 



Where the rock is well exposed it is cjuite common to see a thin seani of sandstone, some- 

 times as much as two inches thick, between two flows. Also where pockets are formed in 

 the top of the flow, as they sometimes are, they are filled with well stratified sand. These 

 things would seem to indicate that these Keweenawan flows occurred under water. 



3. Felsite: Above this thick mass of basic rocks which is almost wholly of eruptive ori- 

 gin is a felsite flow. I shall call it in this paper the Chippewa felsite. It has a dip of 45 °N. 

 and a strike of N.80°E. It is certainly 460 feet thick, and, as there is a space above it with- 

 out outcrop, it may have a thickness of 1,400 feet. But, since there are so many thin strata 

 beginning to appear at this horizon, it would Ije simply a guess to say what rock fills the 

 gap. The felsite may continue across it, the conglomerate above may extend cIo'^ti 

 through it, or it may contain basic eruptives. The felsite is not unlike that composing 

 some of the pebbles that occur in the conglomerates above it. 



There are outcrops of this rock only in Black River and on Chippewa Bluff. In the lat- 

 ter place it forms the cap of, but not the whole hill. For at least two-thirds of the way up 

 the south side there are exposures of ophite, and before reaching the felsite there are some 

 thin beds of sandstone and a small amount of volcanic material. 



