148 METAMOEPHIC ROCKS 



The name phylUte is used by German petrographers to desig- 

 nate a micaceous semi-crystalline rock standing intermediate 

 between the true schists and clay slates. Quartzite is a more 

 or less schistose or banded rock consisting essentially of crys- 

 talline granules of quartz. Such originate from the induration 

 of siliceous sandstones as already explained. 



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

 colored brown and red by iron oxides. At times a greenish tinge 

 is imparted through the development of chloritic minerals; ac- 

 cessory minerals are not, as a rule, abundant. 



The hornblende schists are, as a rule, less finely schistose than 

 are the mica-bearing varieties, owing to the fact that the mineral 

 hornblende itself has not a platy structure. The glaucophmie 

 schists are perhaps the least abundant. Such have been described 

 from the Isle of Syra, in the Mediterranean Sea, Switzerland, 

 "Wales, and Italy ; a more massive form, probably an altered erup- 

 tive, is found near the mouth of Sulphur Creek, Sonoma 

 County, and other parts of California. AmphiioUte is the name 

 given to an extremely tough and often massive rock of obscure 

 origin, consisting essentially of the mineral amphibole or horn- 

 blende. In some instances actinolite and tremolite take the place 

 of the common hornblende. The tremolite rock may undergo 

 alteration into serpentine under proper conditions. Eclogite is 

 a tough, massive, or slightly schistose rock, consisting of a 

 grass-green variety of pyroxene, and small red garnets, with 

 which are frequently associated bluish kyanite, green hornblende 

 (smaragdite), and white mica. Garnet rock, or garnetite, is a 

 crystalline granular aggregate of garnets with black mica, horn- 

 blende, quartz, and magnetite. Kinzighite is a somewhat similar, 

 though fine-grained and compact, rock consisting of garnets, 

 plagioclase feldspar, and black mica, which is found in Kinzig 

 and the Odenwald. 



Many of the rocks of this group are but products of dynamic 

 or contact metamorphism ; this is the case with many of the 

 chiastolite and argillaceous schists or roofing slates. Eocks of 

 the latter group pass by insensible gradations into clastic ar- 

 gillites. They owe their cleavable property to shearing, as 

 already explained. Under the microscope these rocks are 

 found to be quite variable. Hawes described clay slate from 

 Littleton, New Hampshire, as consisting of a mixture of quartz 

 and feldspar, in particles as fine as dust. They contained also 



