HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 



549 



chlorite- schists, talc-shists, hornblende, actinolite, glaucophane schists, etc. 

 The term slate was originally applied to these and other types of rock 

 of schistose or fissile character. In the arrangement here adopted this 

 term is restricted to the argillaceous fragmental or semi-crystalline and 

 foliated rocks next to the described. 



The first of the above mentioned varieties — the mica schists — are 

 represented by some 40 specimens from widely separated localities, 

 principally American and European, the mica being in some cases bio- 

 tite, in others muscovite, or perhaps a mixture of the two. The princi- 

 pal accessories sufficiently developed to be conspicuous are staurolites 

 (specimens 29358, 29359, 36764 and 36822); chiastolites (29361 aud 

 36209) ; garnets (36122, 36881 aud 36882) ; and tourmalines (28574). In 

 the sericite schists the hydrous mica sericite prevails (specimens 36715, 

 36716, and 36718, from Germany) ; Paragonite schist carries the hydrous 

 sodium-mica paragonite (specimen 36720, from St. Gotthard, Switzer- 

 land) ; Ottrelite schist carries the accessory mineral ottrelite (specimens 

 36724, from Ottrey, Belgium, and 37659, from Massachusetts). 



The name phyllite is used by German petrographers to designate a 

 micaceous semicrystalline rock 

 standing intermediate between 

 the true schists and clay slates. 

 (Specimens 36697,36699, 36701, 

 36704, and 36706, from various 

 European localities.) Quartzite 

 is a more or less schistose or 

 banded rock consisting essen- 

 tially of crystalline granules 

 of quartz. Such originate from 

 the induration of siliceous 

 sandstones. This induration is 

 brought about through a de- 

 position of crystalline silica in 

 the form of a binding material 

 or cement around each of the 

 sand particles of which the 

 stone is composed. Each of 

 these granules then forms the nucleus of a more or less perfectly out- 

 lined quartz crystal. This structure is shown in Fig. 95, drawn from 

 a thin section of a Potsdam quartzite from St. Lawrence County, New 

 York. The rounded more or less shaded portions represent the origi- 

 nal grains of quartz sand, and the clear colorless interstitial portions 

 the secondary silica. 



The quartzites consist, as a rule, only of silica, or silica colored brown 

 and red by iron oxides, as shown in specimens 37680, from Sioux Falls, 

 Dakota, and 73078, from Madison County, Montana. At times a green- 

 ish tinge is imparted through the development of chloritic minerals 

 (specimen 73079) j accessory minerals are not, as a rule, abundant, and 



Fig. 95. 



MlCEO-STRUCTU KE OF QUAUTZITE. 



(Potsdam, New York. ) 



