CHAPTER IV. 



By E. D. Irving and C. R. Van Hise. 



THE QUARTZ-SLATE MEMBER. 



Applicability of tlif uaiin'. ficograpliical extent. Topographical features. Thickness. General 

 petrographical character ami stratigraphy. Microscopical character of the feldspathic quartz- 

 » slates. Microscopical character of tbc biotitic ami chloritie quartz-slates. Microscopical char- 

 acter of the vitreiuis ([uartzite. Microscopical character of the samlstone, novaculitc, and argil- 

 laceous slates. Tabulation of petrographical observations. Contacts with the Cherty limestone 

 member. Contacts with the Southern Complex. Change to the Iron-bearing member. Promi- 

 nent exposures. Mode of deposition and source of material. Summary. 



AppUcahility of the name. — Resting directly upon tlie limestone oi- white 

 chert of the formation described in the last chapter, or, in the absence of that 

 formation, directly upon the gneiss, schist, or granite of the Southern Com- 

 plex, follows a set of slaty layers, which, though for the most part less than 

 500 feet in total thickness, constitute a singularly well marked horizon, 

 traceable throughout the entire extent of the area occupied by the Iron- 

 bearing series of the district. The jirincipal ingredients of these layers are 

 quartz and feldspar, which fact has led to oiu" selection of the name Quartz- 

 slate for the entire member; although, as shown subsequently, very many 

 varieties deserving of distinct lithological names occur within it. In the 

 Wisconsin State Reports' the term siliceous slate is made use of for this 

 member of the series, but this term has been so widely applied, particularly 

 in its Grerman form of Kieselschiefei", to a rock whioli is chertv or iionfraff- 

 mental in its nature, that it appears inapplicable in the present case where 

 a fragmental texture is the most prominent characteristic. The possession 



'Vol. in, p. 111. 



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