288 TUE MAEQUETTE IEON-13EAK1NG DISTEICT. 



least-marked !Sta<ie of alteration the quartzites were simply broken to a 

 greater or less amount, and the creA'ices thus formed were cemented with 

 finely crystalline cherty quartz, or with oxide of iron, or both. In a further 

 stage of the process the quartzites were fractured through and through, 

 and in places thev i)ass into reibungsljreccias. In the numerous ramify- 

 ing, bran.ching, and intersecting cracks, silica and iron oxide infiltrated. 

 The silica in jilaces took on cherty or jaspery form.s, and in other places 

 it crystallized as a vein quartz. Tlie secondar)- material may locally be 

 so abundant as to compose a large part of the rock, and rarely considerable 

 belts of chert or -s-ein quartz and iron oxide may be seen. In proportion 

 as the fracturing and the amount of secondary cherty silica increase, the 

 rocks assume a peculiar viti-eous aspect. The iron oxide crystallized as 

 hematite and magnetite, the latter now largel}' changed to martite. 



In their very general brecciation, with consequent considerable areas 

 of pseudo-conglomerates, in the secondary veining, both with coarsely and 

 finely cryi?talline quartz, and in the large quantity of secondary hematite 

 and magnetite, these quartzites differ from the Goodrich quartzite of the 

 Upper Marquette series. Apparently in some cases the brecciation was 

 produced before the rocks became thoroughly indurated, while the frag- 

 ments had a san(h- matrix, in whic-h case the individual grains were 

 broken asunder, and the whole has been indurated by secondary infil- 

 trating silica and iron oxide. In some localities very peculiar dynamic 

 eftects are observable. As a consequence of the folding a most curious 

 spheroidal fracturing has occurred, resulting in roundish pebble-like and 

 bowdder-like forms. Iron oxide has infiltrated along the cracks, and has 

 especially afiected the more fractured and broken matrix, so that the 

 spherical pieces appear like pebbles derived from a difterent rock. In the 

 most brecciated phase we have a pseudo-conglomerate consisting of white 

 spheroids of (juartzite in an iron-stained cpiartzite matrix; a close exami- 

 nation shows, however, that many of the supposed pebbles are not entirely 

 surrounded by the matrix, each being really attached at some place to it. 

 Following along the pseudo-conglomerate belt, we pass from this most con- 

 glomeratic-looking phase to that in which there is less and less dynamic 

 eff'ects, and the rock by gradation passes into the ordinarj' quartzite of 



