THE EASTERN AREA. 405 



contaiiie<l. The matrix of tlie rock is so stroiio- that wlioii broken the fracture passes 

 through the large particles of quartz iustead of tearing them from their sockets. 



The api)earauee iu hand specimen is boine out in thin section. Nearly all of 

 the roun<leil fragments are (piartz, although nungled with them area few small ones 

 of feldspar. Most of these grains are simple, but some of them are very complex. 

 But few of them are enlarged, and these but slightly. Tlie i)lentiful matrix in which 

 they are contained consists of finely crystalline quartz and serieite or muscovite, 

 mingled with a little pyrite. In this rock the induration is as great as in any viti'eous 

 quartzite, yet it is not caused by interlocking enlargements so common in qnartzites, 

 but by the deposition of independent interstitial (piartz. 



■41. Peldsi>athic quartzite, from a middle horizon, north of 40. Specimen 12000 

 slide 5355), 62 N., 1973 W., Sec. IS, T. 47 N., R. 42 W., Michigan. 



The rock is a gray, coarse grained, vitreous quartzite, which breaks with cou- 

 clioidal fracture. 



The thin section differs from that of 40 chiefly in that mingled with the large 

 quartz fragments are many of orthoclase, microcline, and plagioclase. 



Exposure in the NE. ^ of See. 19, T. 47 ¥., B. 42 W., Michigan. 



42. Chloritic and sideritic slate. Specimen 12841 (slide 549S), 1500 N., 250 W., 

 See. 19, T. 47 N., R. 42 W., Michigan. 



The ro(!k is dark green, very flue grained, and finely foliated. The specimen is 

 cut by veins of quartz. 



The mass of the section consists of exceedingly fine material, which appears to 

 be quartz, chlorite, kaolin or serieite (or both), and oxide of iron. Mingled witli this 

 matrix are small grains of quartz and feldspar, which have a strong fragmental 

 appearance. The rock is taken to be a flne grained squeezed clayey one. 



Section in the east part of Sec. 20, T. 47 N., B. 42 W., 3Iichi(fan. 



43. Clay-slate, from a low horizon. Specimens 9206 (slide 2935), 9207 (slide 

 3305), 9208 (slide 299S), 0209 (slide 2930), 7356 (slide 1820), 425 N., 50 W., Sec. 20, T, 

 47 N., R. 42 W., Michigan. 



The rocks are from light grayish to very dark green, aphauitic, and have a ready 

 cleavage. 



The sections are all exceedingly fine grained. They consists of finely crystalline 

 and cherty silica, abundant chlorite, kai)lin, iron oxide, and perliaps other constituents. 

 In one section many of the quartz grains are of sufiQcient size to make it jdaiu that they 

 are fragmental. 



44. Chloritic sericite-slate, from a low horizon, north of 43. Specimen 12002 

 (slide 5.3i-)C), .->50 N., ;!00 \V., Hov. 20, T. 47 N., R. 42 W., Michigan. 



The rock is greenish gray, ai)hanitic, and finely foliated. * 



