478 THE MARQUETTE IHON-BEAKING DISTRICT. 



breccias more closely than do those with tlie schistose or those with the 

 tufFaceous matrix. Their grovmdmass is identical in structure and composi- 

 tion with the rocks constituting the sedimentar}- beds of the formation. In 

 this are embedded tuff fragments, fragments of biotite-schist, occasionally 

 large pieces of an ainphibole rock that rcsendjles a slightly foliated green- 

 stone, pebbles of iron ore, and others of a fragmental rock similar to that 

 forming the matrix. Usually the fragments are not so abundant but that 

 the character of the matrix is easily recognized. In a few cases, however, 

 they are present in sucli large quantities that the existence of any matrix 

 can with difficulty be detected. 



The conglomerates of all classes are more or less clearly banded, 

 and all are foliated in some degree at least. The most schistose phases 

 are those whose matrix resembles a biotite-schist, and the least schistose are 

 those with a sedimentary groundmass. Garnets are common in all of these 

 rocks. They are apparently most alnnidant in those that contain the great- 

 est amounts of basic components. In the conglomerates with the schist 

 matrix theA' are most common. Here the garnets exist as pink granular 

 lines marking the bedding planes of the original rock. 



The microscopical examination of the conglomerates and breccias adds 

 little to the knowledge concerning their nature which is gained from obser- 

 vation of the rocks in the field. The matrices, as has repeatedly been 

 stated, are similar to the materials of the sediments and tuff's that constitute 

 such an imjiortant portion of the entire Clarksljurg fonnation, although 

 in no case are they purely sedimentary or tufiiiceous, as are some of the 

 nonconglomeratic sediments or tuffs. Usually there is an admixture of 

 sedimentary and volcanic material in the groundmass of these rocks as 

 well as in the pebbles embedded in it. 



The schist-conglomerates have a matrix composed of quartz, biotite, 

 hornblende, magnetite, occasionally a little altered plagioclase, and some- 

 times a few crystals of tourmaline. The quartz and Ijiotite are arranged 

 to form a foliated groundmass, through wliicli the other components are 

 scattered. In this schistose groundmass are also the small fragments of 

 quartz, graywacke, quartzite, iron ore, and greenstone already referred to 

 above. The quartz is often in rounded grains, as though waterworn. The 



