406 THE PENOKEE IRON-BEARING SERIES. 



The thin section contains an almost solid mass of minute fibers of chlorite and a 

 mineral which is taken to be sericite, mingled with which are finely crystalline quartz 

 and ferrite. Contained in this background are numerous small i>articles of quartz 

 and feldspar, which are plainly fragmental. The sericite is the moat abundant niin 

 eral of the slide. It has strong absorptive power and gives brilliant polarization colors. 

 The fibers lie with their longer axes in a common direction, and so extinguish simul- 

 taneously. The laminae are in wavy lines, often broken asunder or sharply foliated as 

 if they had been subject to intense pressure, and thus correspond in their appearance 

 to the foliated character of the hand specimen. 



45. Chloritic and sericitic slates, from a middle horizon, north of 44. Specimens 

 12603 (slide 5357), 427 N.,350 W.; 12G05 (slide 5359), 1000 N., 340 W., Sec. 20, T. 47 

 N., R. 42 W., Michigan. 



The rocks are dark green, tine grained, and finely laminated. 12005 is some- 

 what coarser grained than 12G03. 



The thin sections of these rocks are closely allied to that of 44, the chid' differ- 

 ence being that they contain more numerous and larger fragments of feldspar. 



Exposures in NW. I of Sec. 28, T. 47 N., R. d2 W., MicUgdn. 



46. Sericite-schist. Specimens 12816 (slide .5490), 12911 (slide 5515), 1472 N., 

 1499 W., Sec. 28, T. 47 N., R. 42 W., Michigan. 



The rocks are gray, aphanitic, finely foliated, are hard and felsitic appearing. 



The thin sections have a finely crystalline background com])osing two-thirds or 

 more of the sections, which contains roundish and angular grains of quartz of varying 

 sizes. The background appears to be chiefly made up of quartz and sericite, mingled 

 with which is some oxide of iron. The quartz grains, set in a matrix of finely crys-, 

 talline quartz and sericite, at once suggests a coarse fragmental material in a non- 

 fragmental sediment; but the excessive angularity of some of the quartz grains, and 

 the lack of any enlargements, makes the fragmental character of the quartz somewhat 

 doubtfnl, and thus it is difficult to determine whether this rock is a fragmental 

 one or a felsitic schist. 



47. Chert-breccia. Specimen 12625 (slide 5374), 1730 N., 1480 W., Sec. 28, T. 47 

 N., R. 42 W., Michigan. 



This lock consists of angular to rounded fragments of gray and black chert, which 

 vary in size from minute particles to pebbles several inches in diameter. These dierty 

 fragments are set in a matrix, which appears to consist mostly of the same material, 

 in which occur large simple grains of vitreous quartz and cleavage areas of feldspar. 



In general the section as seen under the microscope covresitoiids in a])pear- 

 ance in hand specimen. The large fragments of the section are all (iiicly crystalline 

 cherty silica, the outlines of which are only discoverable by using a low |)o\ver. 

 Two of these fragments are somewhat peculiar. One ccnitains besides tlie finely 



