THE UPrER SLATE MEMBER. 307 



rock is u.sually given by "siliceous paste," altliou<^li much of the cemeut is 

 frequently in the form of quartz added to the rounded fraj^mental grains. 

 There is frequently " argillaceous and calcareous" and occasionally car- 

 bonaceous material present as accessories. There is also found in all of 

 these graywackes abundant chlorite or mica, or both. The mica is chiefly 

 biotite, although muscovite or sericite, or both, are plentiful. In impor- 

 tance, these minerals are secondary only to quartz and feldspar. The class 

 of locks in this district — graywackes and graywacke-slates — has been 

 divided into three phases — micaceous, micaceous and chloritic, and chlo- 

 ritic. These divisions have a geographical significance as well as a litho- 

 k)gical one. All of the graywackes and graywacke-slates in Michigan and 

 Wisconsin as far Avest as the Potato river are chloritic ; the Potato river 

 rocks are mostly micaceous and chloritic, these two minerals being in about 

 equal quantity ; about half of the rocks in the vicinity of Tylers fork are 

 both chloritic and micaceous; the remaining half micaceous only; west of 

 Tylers fork and vicinity the graywackes and graywacke-slates are mica- 

 ceous only. We have thus a series of graywackes which at the east end 

 are wholly chloritic ; in passing to the west mica appears with the chlorite, 

 becomes more and more plentiful, and ultimately entirely replaces the chlo- 

 rite. How gradual and complete this transition is will be readily seen by 

 turning over the taljulations of these rocks on subsequent pages. The 

 graywackes and graywacke-slates are so closely associated, both strati- 

 gra}»hically and litliologicalh', with the mica-slates and mica-schists, that a 

 more detailed characterization of them will be deferred initil the general 

 character and distriliution of tin- latter are given. 



The mica slates and iinca-sclmts^ (Pis. xxxiii and xxxiv) of tlie district 

 have always as a cliief constituent quartz, and usually this mineral is the 

 most al)undant of all. Feldspar, as in the graywackes, is almost always 

 jireseut, but its quantity is nuich less, and In the typical slates and schists 

 is almost wholly ab.sent. Biotite, nuisco\ i-. sericite, and rarely chlorite, 

 or two or more of these minerals, are always plentiful, the individuals 

 generall)- being in well defined folia. The accessories present are the same 



' .Vlthougli in ono placr clilnritc is. iisiili- from the tVldspar, tlm chief ciiiistitiiriit, they will all 

 be called uiica-schists and miia-slatus, U> distiujjuish thciu from the graywackes aud graywacke-slates. 



