THE QUAKTZ-SLATK MEMREK. 1()5 



A breccia composed chiefly of wliife, gmy, V)lack, and red chert, but containing' 

 also fVafifineiit,s of other minerals and of granite, nnd resting directly upon the 

 gi-anite. 



The thin sections are made up mainly of rounded fragments i>i' chert included 



iu a matrix of the same material. This chert, in both fragments and matrix, consists 



of an exceedingly minutely crystalline quartz, with possibly some intermingled 



aiuori)lious silica. Tliere are also included in the matrix rounded i>articles of quartz 



and fehlspar, areas of chlorite and of some carbonate, and complex fragments of 



granite. 



From the Black river section. 



34. Chloritic and cherty slates and quartzites, from a middle horizon. Speci- 

 mens 94G0 (slide ;}078), from 1445 N., 1185 W.; 94G1 (slide aUO), from 14G0 N., 1185 

 VV.;94Gl' (slide ;$(»79), from 1475 N., 1185 W.; 94G.'. (slide 3080), from 1490 N., 1185 

 W., Sec. 13, T. 47 N., R. 4G W., Michigan. 



These specime*is rejiresent mterstratifications of slaty and quartzitic phases. 

 The slaty phases are flue grained, in places green, in i)laces brown, with variations 

 from one color to the other. The quartzitic layers are gray and vitreous. 



In thin .sections th(> shity and quartzitic jihases differ from each other only in 

 coarseness of grain and proportions of the ditferent mineral constituents, the diief 

 ones of which are (juartz, fehlspar, and chlorite, the two former being plainly in a 

 fragmental condition. In 3079 the quartz fragments are greatly preponderant and 

 widely enlarged. In the slaty portions, mixed with the small (juartz grains, are many 

 fragments of a minutely complex silica, which at times is in part probably even amor- 

 phous or opaline. The chlorite is particularly plentiful in the slaty phases. (PI. 

 XVIII, Fig. .3.) 



35. Sandstones, tr-om a high horizo'i. Specimens 9512 (slide 3088), 9513 (slide 

 3089), from 1510 N., 1950 W.; 7557^ (slide 19SG), from 1570 N., 1950 W., Sec. 13, T. 47 

 N., K. 4G W., Michigan. 



These are fine grained, arenaceous rocks, having strangely marked sedimentation 

 laminae Some siiecimcns are light greenish gray, others are dark brown; while iu 

 still others are found both colors arranged in the most irregular manner. 



The thin sections are composed mainly of medium sized fragments of quartz and 

 feldspar. The quartz fragments are mostly from single individuals, and have gener- 

 ally received secondary enlargements, but some of them are composed of finely 

 crystalline cherty silica. The rather plentiful cementing material ai)i)ears to be a mix- 

 ture of minutely crystalline (piaitz with Hakes of kaolin, chlorite, and brown iron oxide, 

 the browner portions, as seen nnicroscopically, differing from those that are of paler 

 color only iu containing a larger quantity of the iron oxide. 



From tlie Sunday lake section. 



36. Graywacke-.slates (feldspathic quartz-slate), from a middle horizon. Sjteci- 

 meus 9442 (slide 3071), from 3G0 N., 1000 W.; 9443 (slide .•!(I72), from 325 N., 10(10 VV., 

 Sec. 10, T. 47 N., K. 45 W., Michigan. 



