THE QITAKTZ-SLATE MEMBER. 



155 



The arg-illacenus slates (PI. xix, Fig. 4) occur in a immlier of places, but 

 nearly always in relatively thin seams, intei'stratifieil with the coarser vari- 

 eties into which they gi'ade. These slates appear to diifer from the feld- 

 spathic (|uartz-slates merely in having- the coarser fraginental portion almost, 

 although not qiiite, excluded; that is to say, thev are of the same nature 

 as the matrix portion of the feldspathic (luartz-slates, with ])erhaps an un- 

 usual amount of kaolinitic material from the alteration of feldspar detritus. 



Tabulation of jHirof/rapliirtd observations. — The following tabulation ex- 

 hibits a series of observations made on specimens from this formation. As 

 in the case of the limestone formation, these observations are tabulated in the 

 first place in g-eograpliical order, proceeding from the west to the east. In 

 the second place in stratigraphical order for each cross-section examined, 

 beginning \Aith the lower horizons in each case and proceeding to the higher. 



From the .wtiov in Sec. 34, T. 44 N., E. -I W., Wisconsin. 



1. (.'hloiitic and biotitic slate, from a miiUlle horizon. Speciiiicus 904.^ (.slide 

 ;}15o), DGtO (slide ;5IoG).' From ir.OO N., W., Sec. 24, T. 44 N., II. 4 W., Wisconsin. 



A finely g'rannlar, dark gray, nnit'orndy textured material, of almost qnartzitic 

 (•oin])actness, is interbanded with a finely laminated material of a greenish gray color, 

 which U|>on the surface of the lamina^ shows the sheen of mica. 



The sections consist chiedy of quartz, feldspar, kaolin, chlorite, and biotite, the 

 tirst two minerals being fragmental. In the more qnartzitic pha.ses the quartz in 

 rather small uniform sized fraguuMits, some of which have received a secondary en- 

 hugement, constitutes two-thirds of the nurss of the rock. The feldspar fragments 

 include l)oth orthoclase and plagiodase. Scattered through the mixture of quartz and 

 feldspar, aud composing the greater part of the remaining third, are small flakes of 

 chlorite and biotite, the former the nu)rc abundant of the two. The chlorite is in well 

 defined Hakes which extinguish rectangularly. The schistose part of the rock con- 

 tains a greater proportion of feldspar, kaolin, and biotite than the more rpiartzitic por- 

 tion. The biotite and clilorit*' both appear to be secoinlary developments from the 

 feldspar fragments. Magnetite is an unimportant accessory. 



2. Hornblendic and chloritic quartzites, from the uppermost layers, above 904.') and 

 9C4G, and immediately underneath the Iron-bearing member. Specimens 9047 (slide 



'The numbers of specimen.s and sliiles are usually those of the pollection of tho lake Superior 

 division. (Specimens with Wr. after the numbers are from the collection of the late Mr. Charles E. 

 Wright. Specimens with Wis. after the numbers are from the collection of the Wisconsin Geo- 

 logical Survey. Locations are given from the southeast corner of the sections, in steps of 2,000 

 per mile. 



