538 THE MAEQUETTE IRON-BEAEING DISTEIGT. 



SECTION IV.— LATER IG:N^E0US INTBUSIVES. 



These are the diorites of Brooks, and they occur in great abundance 

 in both the upper and h;)^yer series. They are dark-green to bhick, often 

 coarsely crystaUine rocks, composed essentially of green hornblende, biotite, 

 and plagioclase, and doubtless were originally diabases. They occi;r in 

 sheets intruded parallel to the stratification of the bedded rocks, in dikes, 

 and in irregular bosses. The great regularity of some of the intruded 

 sheets, such as those on Republic Mountain, is remarkable, and led Brooks 

 to regard them as regularly interbedded and continuous members of the 

 stratified series. Close examination, however, shows that even here they 

 often reallv traverse the banding of the iron-bearing member at small 

 angles or in steps. In one case a dike several feet wide was found to leave 

 the main sheet and to cut the structural planes of the inclosing jasper at an 

 angle of 45°. In the immediate neighborhood of the ore deposits bodies 

 of so-called soaprock are found, which have in many cases intrusive rela- 

 tions to the iron-bearing member. At Republic it was not possible to follow 

 these soaprock bodies in any instance into a rock which retains traces of 

 an original crystalline structure, l)ut at the Champion mine exacth^ similar 

 soaprock, occurring in similar relations to the ore, in several instances was 

 found to run into tvpical diorite. 



In age, many, probably most, of these rocks are younger than the 

 Upper Marquette sediments. Some, however, penetrated the Lower Mar- 

 quette series before Upper Marquette time. In sec. 23, T. 47 N., R. 31 W., 

 the basal conglomerate of the Upper Mai-quette series is seen to rest upon 

 and to hold numerous fragments of an old diorite. Within the Republic 

 area no surface eruptives have been seen in either the upper or the low^er 

 series. 



SECTIOX v.— GENERAL GEOLOGY. 



All the rocks of the Upper Marquette and Lower IMarquette series 

 have been closely folded in the Republic area into a syncline the axis of 

 which runs about northwest and southeast. The present fold for most 

 of its length is sunk deeply into the Archean, and the axis is practically 

 horizontal. Southeast of Smiths BaA', however, the axis rises with a pitch of 



