228 THE MARQUETTE lEON-BEAEING DISTRICT. 



iireat dciil of secondary clierty quartz deposited. Also numerous veins of 

 secondary- clierty quartz are present. In these are inclosed fragmental 

 grains derived by dynamic action from the original material. In some 

 cases the vein material passes gradually into the ordinary rock, more and 

 more of the original fragmental quartz appearing, until the grains are 

 merely broken apart, with secondary quartz between them. 



In one phase of the clierty quartzite it appears that after the rock had 

 been subjected to a first dvnamic movement and had been cemented by 

 «herty quartz a later dynamic movement broke up this rock along certain 

 zones, thus producing reibungsbreccias, the fragments of which are com- 

 posed of simple grains of quartz, mingled with chert grains. The whole 

 was afterward cemented by a later infiltration of silica and oxide of iron. 

 This phase suggests that many of the chert fragments, and possibly some 

 of the ferruginous chert and jasper of the (piartzite, were jjroduced by 

 dynamic processes operating upon a rock which had been previously 

 Ijroken and cemented by secondary chert and jasper. 



The conglomerates, in their passage to the quartzites, at many jjlaces 

 grade through tlie phase of grmjicackes or (/rai/wacke-slafcs, and the quartz- 

 ite also o-rades above into similar rocks. These graywackes have a clayey 

 l)ack<>-round, in which are set many small and medium-sized, well-i-ounded 

 to subangular grains of quartz and the various feldspars. The feldspars 

 are frequently altered in part to kaolin, sericite, and quartz. Occasional 

 complex o-raius of clierty quartz are seen. The matrix consists of finely 

 crystalline quartz, kaolin, and sericite, with occasional large flakes of mus- 

 covite. In many places it is stained with iron oxide. In the rock 

 which has suffered the least from dynamic action, undulatory extinction and 

 fracturing are seen in the grains of quartz, but the pressure has not been 

 sufficient to ffive a distinct arrangement of the particles Avith their longer 

 axis in a uniform direction. 



In a more mashed phase of the graywackes the quartz and feldspar 

 iiarticles .show a distinct arrangement with their longer axes in a common 

 direction, and, with this, most marked undulatory extinction, fracturing, and 

 even granulation. Some of the larger grains show particularly well the rect- 

 auo-ular fractures in two directions spoken of in connection with the quartzites. 



