r 



GENERAL (lEOLOGY OF THE DISTRICT. 443 



they are seen to be of completely crystalline kinds. Sncli massive rocks as 

 the granites and syenites are now regarded as eru])tivi'. The origin of 

 many of the thinlv foliated schists and gneisses is not known. It is enongh 

 here to say that, as indicated (pp. 125-126), nowhere in this area is there 

 any sufficient evidence to show that anv of them ari^ of a fragmental char- 

 acter. Certain of the gneisses of this area have sncli lelations with the 

 massive syenites and gi'anites that they may be considred as of eruptive 

 orig-in; so that all that ciui he said as to the orio-innl condition of these 

 rocks is that a large ))roportion of them are eru])ti^•e A\liile the origin 

 of others is unknown. The rocks to the soutli of the Penokee series are, 

 then, a set of massive rocks and crvstalline schists ha\ing a peculiar com- 

 plex contact and neither certainly possessing a sedimentary structure of 

 any kind. This Southern Complex, as a whole, may be taken as the base- 

 ment group of the district. 



The Clierti/ Jlmestonc and Quartz-slaie viemhers. — To the north of this 

 complex are the belts of the Penokee series and the riicrty limestone. In 

 chapters in to viii these rocks have been described in detail. Here it will 

 be necessary to recapitulate such of the facts as bear upon their relation to 

 another and their relation to the rocks to the south and north of them. 

 The lowest formation is a cherty limestone. This rock is found in large 

 exposures at various places from near Atkins lake, T. 44 N., R. 5 W., Wis- 

 consin, to T. 47 N., R. 44 W., Michigan. In its greatest develojiment it is 

 about 300 feet thick. It appears to be a tolerably contimious foiination 

 throughout its western portion, but in the eastern half of the disti-ict it is 

 found only here and there. Next in order to the noitli is the Quartz-slate 

 member. This formation is a fragmental one in wliicli the particles are 

 chiefly quartz and feldspar. In the vicinity of the cherty limestones it fre- 

 quently carries a considerable quantity of material from this member, even 

 becoming at times a conglomerate. This cherty limestone and the slate belt 

 are, however, in apparent conformity with each other, so that while there 

 certainly was an erosion interval between the two there was found no evi- 

 dence that the Chertv limestone was closely folded. The case is analogous 

 to that of the relations of the Penokee and Keweenaw series. It will be 

 shown that such an erosion interval implies a considerable physical break. 



