GEOLOGICAL EXPLORATIONS AND LITEKATUliE— 1881. 83 



Romiug-er thinks Brooks's sulKlivisioiis ure more numerous tliaii is 

 necessary in discussing the formations present in the Marquette series. He 

 recognizes only six "groups," as foUows, beginning with tlie h)werniost: 

 the granitie, the ihoritie, the iron, the (juartzite, the arenaceous shite, 

 and the mica-schist. The members of the "granitic grou)).-" are confined 

 to the northern and southern Umits of tlie Marquette area, where thev exist 

 as the predominant rocks, intimately associated>with diorites, green schists, 

 etc., with which they often seem to be interlaminated. 



The granites on both sides of the synclinal basin are similar in compo- 

 sition and structure. They consist essentially of red ortlioclase, quartz, 

 and a micaceous mineral that often resembles chlorite. S(imetimes this is 

 replaced b}^ a hydromicaceous substance which imparts to tlie <i-ranite a 

 subschistose cleavage. Such granites are found at a few i)laces in the 

 northern granite belt and on the south side of the synclinal hasiu, on 

 the line of contact between the normal granites and the lower (juartzite 

 formation. Here they appear to be metamorphosed cpiartzites, "as we find 

 all degrees of transition from tlie ordinary (piartzite into a regulai- granitic 

 rock mass" (p. 15). 



Associated with the granites are also belts of gneissoid rocks, consisting 

 of dark mica or a dark-green hornldeiide, feldspar, and ([uartz. 



This stratified banded rock, in contiguity with the granite and alternating with 

 it in parallel belts, often becomes completely intermingled and entangled with it. The 

 granitic masses intersect the gneissoid, enter wedge-like between them in the direc- 

 tion of the lamination or transversely, inclosing strips of the gneissoid ledges 

 between the loops of the anastamosing granite seams, and, moreover, frequently the 

 so-intermingled masses are curved into the most curious coils and serpentine flexions, 

 ■which evincss their almost liquetied, plastic condition at the time their intermixture 

 took place. (Pp. 16-17.) 



The granites and gneisses are cut by large and small dikes of horn- 

 blende-rocks, by diorites, and by seams of quartz. In the northern halves 

 of sees. 20, 21, 22, and 23, T. 48 N., R. 2(i W., is a range of genuine sye- 

 nitic rock composed of dark hornblende and a reddish-gray ortlioclase. 



From these descriptions it is seen that the granite is considered an 

 eruptive rock intruding certain dioritic and hornblendic schists that must be 



