THE UPPER SLATE MEMBER. 343 



mental, as is evidenced l)y seeondary enlai"j>-einent. Tlio biotitc is all 

 believed to be due to the alteration of feldspar, niucli of it certainly Ijeing 

 of this nature. The matrices of the different sections vary in coarseness 

 and in the relative pro])ortions of the various mineral constituents, l)ut are 

 alike in all essential points. 



We have here plainly a series of rocks which are parallel to (4), 

 the chief diflFerences betvi'een the two sets of rocks being' that the black 

 mica-slates are much finer grained, that they contain UK^re p^rite, and also 

 contain carbonaceous material. As the unaltered or partly unaltered feld- 

 spars are a§ large fis in (4), it is probal)le that the greater fineness of grain 

 in the black mica-slates at present is simply due to the fact that the feld- 

 spars altered to smaller individuals of mica and quartz in them than in (4). 



Sonne of material. — It will be later shown that between the Penokee 

 series and the Southern Complex there is a great structural break, the 

 clastic series having been laid down b)- water action upon the older crys- 

 talline formations. From the description of the Upjjcr slate, it is evident 

 that tlie most of its material has been derived from a set of acid rocks, its 

 two predominating constituents being quartz and acid feldspar in various 

 degrees of comminution. There is no reason to believe that the Southern 

 Comple.x, which stretches a great distance southward, was ever covered 

 thniughout the whole of its extent bv this newer series; and as the mate- 

 rial of which this upper belt is composed is of precisely the character which 

 would be expected'if derived from its granite, gneiss, and schist, it ma^' be 

 considered as probable that this is the source of the matej'ial of the 

 Up))er slate. Tins hypothesis receives reenforcement when the distribu- 

 tion of the granitic and schistose areas is considered, and the material which 

 they are able to su])])ly is comjiareil with that of the material of the U])])er 

 slate meml)er at adjacent points. With the exce})tion of a few miles the 

 newer series of rocks is directh' in contact with areas of green schists 

 as far west as I'enokee gaj). ^^^ithout doul)t the contiguous granitje in 

 the western part of Michigan and south of tlu^ schist areas has also 

 furnished its quota of material ; so that it has come, not from the 

 schist alone, or the granite or gneiss alone, ])ut from both of them. In 

 passing westward toward Bad river, as has been seen, there is a 



