GNEISS 14:7 



sory minerals the schists are particularly rich. The more 

 common of these are feldspar, garnet, cyanite, staurolite, 

 tourmaline, epidote, rutile, magnetite, menaecanite, and pyrite. 

 Through an increase in the proportional amount of feldspar the 

 schists pass into the gneisses, and through a decrease in mica, 

 hornblende, or whatever may he the characterizing mineral, 

 into the quartz schists, in which quartz alone is the essential 

 constituent. Occasional forms are met with quite lacking 

 in quartz and other accessory minerals and consisting only of 

 schistose aggregates of minerals of a single species, as is the 

 case with the pyrophyllite schists (or, more properly, schistose 

 pyrophyllites) from North Carolina, talcose schists, and with 

 the more massive *'soapstones." 



The rocks of this group are characterized as a whole by a 

 pronounced schistose structure, due to the parallel arrangement 

 of the various constituents, this structure being most pro- 

 nounced in those varieties in which mica is the predominating 

 accessory mineral. They are ordinarily considered as having 

 originated from the crystallization of sediments, and in many 

 cases the microscope still reveals existing ''traces of the origi- 

 nal grains of quartz sand and other sedimentary particles of 

 which the rocks at first consisted.'* Like the gneisses, they 

 are in part, however, mechanically deformed massive rocks and 

 their schistosity in no way relates to true bedding, as has been 

 already noted (p. 144). 



The varietal names given are dependent mainly upon the 

 character of the prevailing ferro-magnesian silicate. "We thus 

 have mica schists, chlorite schists, talc schists, hornblende, actinol- 

 ite, glaucophane schists, etc. The term slate was originally 

 applied to these and other types of rocks of schistose or fissile 

 character. In the arrangement here adopted this last term is 

 restricted to the argillaceous f ragmental or semi-crystalline rocks 

 next to be described. 



Of the above-mentioned varieties the mica schists are the 

 most common and widely distributed, the mica being in some 

 cases biotite, in others muscovite, or perhaps a mixture of the 

 two. The principal accessories sufl&ciently developed to be con- 

 spicuous are staurojites, chiastolites, garnets, and tourmalines. 

 In the sericite schists the hydrous mica sericite prevails; para^ 

 qonite schist carries the hydrous sodium-mica paragonite; ot- 

 trelite schist carries the accessory mineral ottreliie. 



