LITHOLOGY OF THE SERIES 361 



in the former of rutile needles, bands of liquid or empty inclusions, a 

 wavy extinction, and other quite general characters. Hornblende is 

 present as a secondary constituent, distributed in fibrous individuals 

 within the matrix enveloping and gradually absorbing the clastic grains, 

 both fine and coarse. 



The feldspars are next to command interest. Rarely are the individ- 

 ual grains of their original size and contour. The outline is not sharp 

 and smooth as one would expect in a rounded grain, but the contact 

 line shows a finel}^ crystalline interlocking of several mineral species. 

 A slight kaolinization characterizes the freshest feldspars, and this in- 

 creases with the degree of alteration undergone. Complete replacement 

 has undoubtedly taken place in some of the feldspars. Figure 1 shows 

 their usually partially altered condition. The grain is still present in 

 spots and lines which lie with considerable regularity of direction. The 

 finely crystalline alteration product in this case is a uniaxial mineral 

 and is thought to be siderite. 



The matrix now binding these grains of quartz, feldspars,, and less 

 frequent rock and mineral fragments together is very finely crystalline. 

 It consists largely of quartz, yet partly of hornblende, biotite, and kao- 

 linic and chloritic minerals. In origin it is la-rgely interstitial deposited 

 as independent particles. Its source is doubtless within the rock mass 

 itself and for the most part in the corroded grains of quartz, feldspar, 

 and other more easily soluble silicates. Not all the matrix material is 

 secondary, since naturally many fine grains of the constituent minerals 

 were sifted in among the larger ones as the rock was laid down. 



The quartz and feldspar fragments even in the least altered sections 

 show no clear traces of secondary growths. In this respect they differ 

 from these same mineral grains in the Penokee-Gogebic range as de- 

 scribed by Van Hise.* 



THE GRA Y WA CKE SLA TES 



The most conspicuous difference between these rocks and those just 

 described is that of texture. The gradation from the coarsest gray wackes 

 to the fine and often glossy graywacke slates is difficult to follow because 

 so imperceptible. There is nowhere any sharp line of separation. Even 

 where the division can be located within a band 2 or 3 inches wide the 

 line is still indistinct. The actual separation is to be sought for in the 

 texture of the sediments out of which these rocks have been developed. 

 The deposits were alternatingly coarse and fine. Where coarse, the gray- 

 wackes have come down, and where fine, the graywacke slates occur in 

 increasing fineness until the clay slates, which have been quarried in 



*Loc. cit., p. 456. 



