308 THE PENOlvEE IRON-BEARING SERIES. 



us ill the graywackes — ferrite, pyrite, some cai'bouate, and rarely carbona- 

 ceous matter. xVs biotite is usually the prevalent mica, the slates and 

 schists have as a whole a darker color than the gray vvackes, ranging- from 

 very dark gray to black. The fundamental ditferenees, however, which 

 separate them from the graywackes are their crystalline appearance and 

 absence of feldspar. Very many of them when viewed under the micro- 

 scope, unless closely examined, taken by themselves, show no trace of 

 a clastic origin; although the greater number, ujion closer examination, 

 are' seen to have something of a fragmental character. The distinction 

 made between mica-schists and mica-slates is based upon structure and 

 coarseness of grain. Most of the coarser grained upper member mica rocks 

 have a well developed schistose structure parallel to the lieddiiig of the 

 rock — a structure which could fsxirly be called foliated in some places. They 

 never become very thinly foliated, or contorted with a brilliant sheen upon 

 the foliation surface, as do some (tf the most crystalline mica-schists. A 

 few exposures have a dark and liglit banded appearance, suggestive of a 

 fine grained gneiss, which in fact the}' are when the feldspar is a chief con- 

 stituent. The rocks called mica-slate;^ are generally finer grained than the 

 mica-schists ; they cleave with n very smooth slate-like parting parallel to 

 the bedding. Many of them have a Ijlack color, due to abundant dark 

 biotite, particles of ferrite, and in some places to carbonaceous material. 

 Like the graywackes and graywacke-slates, the mica-slates and mica-schists 

 have been divided into three divisions: those in which chlorite is the chief 

 constituent aside from the (piartz and feldspar; tlio.se in which mica takes 

 tliis place; and those in which both chlorite and mica are abundant. Tlie 

 first division is represented by but a single exposure. The chloritic nnd 

 micaceous schists and slates are quite plentiful, l)ut by far the greater num- 

 ber of the slates and schists are micaceous only. As with the graywackes 

 and graywacke-slates, their classification corresponds to geographical dis- 

 tribution, and the distribution is of the same soi't as that of the graywackes 

 and graywacke-slates. The one exposure of sericitic chlorite-schist is but 

 a short distance west of the Montreal i-iver, in Sec. 14, T. 4(i N., R. 2 E., 

 Wisconsin. In ])assing to tlie west, the micji-slatcs at tlic Potato riv(^r first 

 appear. At tliis locality only one exposure is known which falls under 



