MICHIGAN ACADEMY OF SCIENCE. 193 



warrant any very general conclusions, bnt there are a few points that 

 may be noted. 



There are no very thick flows in the Keweenawan as it is exposed 

 in this section, nor are all of the flows alike lithologically. In the 

 great eruptive division below the Chippewa felsite there are melaphyrs, 

 ophites, labradorite-porphyrites, and felsites. and, indeed, almost every 

 degree of acidity is displayed from the labradorite-porphyrites up to the 

 felsite-porphyrites. All of the flows have amygdaloidal surfaces and 

 massive centres, and most of them have pipe amygdules at the base. 



Keweenawan time seems to have been inagurated quietly as is indi- 

 cated by the basal sandstone. Soon, however, it displayed itself more 

 violently, and lava flow after lava flow followed one another building 

 up a great column of eruptives. Some of these flows seem to have fol- 

 lowed each other in rapid succession, and again, a little time seems to 

 have elapsed as is indicated by the thin seams of sandstone between 

 some of the flows. The great eruptive period did not end as suddenly 

 as it began for the sedimentaries become gradually more and more 

 prominent as we ascend in the series. This prominence is displayed in 

 the increasing frequency and greater thickness of the beds. 



It is a very striking fact that all of the sedimentary rocks save the 

 Nonesuch, no matter whether above the Chippewa felsite or below, are 

 strikingly acid, and all resemble each other very much. The great mass 

 of the building material of the aqueous rocks is felsite. The Nonesuch 

 is basic and it is worthy of note that there is a sudden change from an 

 acid sandstone to the basic Nonesuch. 



At the base of the Keweenawan there is a very high dip, about 70° N. 

 As we go toward the lake it gradually decreases so that at the mouth of 

 the river there is a dip of not more than 25° N. The strike does not 

 materially change, lying always between N. 70° and 80° E, 



It does not, on the whole, seem that this change of dip is the result of 

 tilting during Keweenawan times. If so the basic rocks would surely 

 take a more important place in the building up of the great conglomerate 

 beds. It is quite true that the basic pebbles would wear down rapidly, 

 but, at any rate, the flner material between the pebbles would be basic; 

 but it is not, it is acid. There are, however, a few of these basic pebbles 

 present, but it seems to me easier to explain the presence of these peb- 

 bles without the tilting in Keweenawan time than it is the absence of 

 basic material if the tilting took place during Keweenawan times. 



The amount of felsite in the sedimentary rocks is enormous when 

 compared with the amount in place. It seems quite certain that all 

 of this material could not come from the felsite beds in sight. It seems 

 more probable that there were originally great felsite knobs that have 

 been worn away. 



Again, there must be a reason why so much basic material began sud- 

 denly to be laid down at the beginning of the Nonesuch formation. It 

 is at least suggestive of a time break, and that the base of the Keweena- 

 wan is being exposed and subjected to degredation. 



Before leaving the rock descriptions I wish to call attention again to 

 the fact that the divisions made above are merely for the sake of con- 

 venience in this present paper. The name 'Thippewa" felsite is used 

 in the same manner. 



In the first part of the paper great north and south gaps were spoken 



