IMIE (,)rAi;T/ SLATK M KMIiKlf. 149 



belt, thono'li steadilv lessening' in relative amount as one moves eastward. 

 East of Sec. 34, T. -if) N., II. 1 W., Wisconsin, although the uppermost 

 (piartzite c<»ntinues well marked, the cross section becomes varied, t'eld- 

 s[)athic ([uartzites, feldspathic quartz-slates, sandstones, and clav-shales alter- 

 nating with one another. 



Microsrnplcal character of the fclihpatJi'ic (jiKirfz-slatcs (Phase 2, see PI. 

 xviu, Fig.s. 1, 2, 3, 4). — In thin sections, from the t\pical specimens of this 

 phase, which is the prevailing- rock of all that ])ortion of the (p;artz-slate belt 

 to the east of Sec. 34, T. 41') N., R. 1 W., Wisconsin, a single glance 

 through the microscope generally suffices to sliow that it is composed of 

 two ])arts, a coarser plainly fragmental pi^rtion and a iiner interstitial 

 material. The relative proportions of these materials var\- great])-, the 

 coarser jjortion at times sinking to quite a subordinate position, and again 

 nearly excluding the matrix. Between the finer and coarser portions there 

 is often a material of an intermediate fineness, and the whole appearance 

 suggests that the two portions are in large measure only finer and coarser 

 particles of the same nfinerals. 



The coarser portion in these sections always comprises fragments of 

 both quartz and felds|)ar. With these is very often more or less rather 

 coarse grained mica, which appears also in a finer condition in the inter- 

 stitial material. This coarser grained mica is taken to be in the main frajj- 

 mental also, although, as will be seen subsequentlj', secondary micaceous 

 minerals are plentifully developed in the rocks of this member. These 

 coarser grained micas are the ones which appear as Iji-illiant flakes to the 

 naked eye on the surfaces of tlie laminic. There is considerable variation 

 in size among the coarser pieces, which, as already said, grade down- 

 ward into the matrix material itself This mingling of coarser and finer 

 material, since it is made up of fragments of diff'erent nfinerals, is taken 

 to indicate that the detritus of which this rock is conqjosed had received 

 relatively little sorting before deposition. 



The fragments of quartz are for the most part jjortions of single indi- 

 viduals, but not unfre(piently the}' are minuteh' complex, having been 

 derived from some chei'ty or fiinty rock, l^hese jiarticles \:\r\ greatly as' 

 to the degree of rounding which they have received. In general the 



