THE UPPER SLATE MEMlJEli. 33 1 



The lock is graj', of a ratlier lino uniform grain, and massive. 



in tlie thin section, fragments of (luartz and feldspar of varying sizes comjjose 

 twotliirds or more of tlie ro( k. Tlic fel<lspar fragments aif often <|uite fresh and 

 comprise ortliochise, mierodine, and plagiochise. The filling material is finely crys- 

 talline silica, chlorite, kaolin, and ferrite. 



7l'. Chloritie graywacke, from a lower middle horizon. Specimen 9517 (slide 

 2980), 325 N., 715 W., Sec. 10, T. 47 N., E. 46 W., Michigan. 



The rock is gray, rather fine grained, massive, breaks with subcouchoidal frac- 

 ture, and contains large fragments of black cherty material. 



In thin section, rather large fragmental particles of quartz and feldspar, the two 

 minerals being in about equal aljundance, compose the greater part of the rock. 

 The grains of (juartz are usually quite widely enlarged. A portion of the feldspar is 

 relatively fresh, but the greater part of it is much cldoritized and kaolinized. The 

 interstitial material consists of quartz, chlorite, kaolin, fi'rrite, with some biotite and 

 mu.scovite. 



73. Micaceous graywacke-slate, from a lower middle horizon. Sijecimen 9518 

 (slide 3090), 325 N., 715 W:, Sec. 10, T. 17 N., R. 46 W., Michigan. 



The rock is dark gray, fine grained, finely laminated, and the cleavage siu'face 

 shows numerous glittering flakes of mica. 



In thin section, rather small clastic particles of quartz and feldspar compose 

 about one-half the bulk of the rock. The grains of quartz are generally enlarged 

 and the feldspars are usually much decomposed. The interstitial material consists of 

 quartz, chlorite, biotite, muscovite, and ferrite. Folia of muscovite and biotite are 

 arranged with their longer axes in a common direction, as though the rock had been 

 subjected to squeezing. 



74. (Ihloritic clay-slate, from a low horizon. Specimen 9515 (slide 2985), 250 N., 

 1750 W., Sec. 11, T. 47 N., R. 46 W., Michigan. 



The rock differs from 69 oidy i)i that its cler.,vage and bedding correspond. 

 The thin section is like that of 6!). 



75. Olay-slate, from base of formation. Specimen 9493 (slide 44l'2), 700 N., 1015 

 W., Sec. 12, T. 47 N,, R. 46 W., Michigan. 



The rock is dark olive green to black, aphanitic, finely laminated, cleaves par- 

 allel to the bedding and also in another direction, cutting across the first at an obtuse 

 angle. Included aie particles and nests of pyrite. 



The thin section is excessively fine grained. It appears to consist of finely crys- 

 tallized quartz, partly amorphous silica, chlorite, feldspar, ferrite, and crystals of 

 jiyrite. Quite a proportion of the ((uartz and feldspar are, however, coarse enough to 

 show that th<^ rock is a fragmental one. 



