HORNBLENDE SCHIST 297 



it sometimes has such an origin, and in certain instances the crushed 

 pebbles of the parent conglomerate are still distinctly visible. Still 

 another series of these rocks are of complex origin, granitic mag- 

 mas being injected along the foliation planes and into all the 

 crevices of metamorphosed sediments. 



Gneisses are widely spread in ancient formations, especially in 

 the most ancient of all, and they cover vast areas in the northern 

 part of North America. 



The Crystalline Schists are more finely foliated than gneiss, into 

 which they often grade imperceptibly, having very similar miner- 

 alogical composition. They have very diverse modes of origin 

 arising from both sedimentary and igneous rocks. Slates, impure 

 sandstones and limestones, as well as felsites, andesites, diabases, 

 tuffs, etc., may all give rise to crystalline schists by thermal or 

 dynamic metamorphism. The varieties are named from their 

 most important ferro-magnesian mineral. 



Quartz Schist is a foliated quartzite in which cleavage or flssility 

 has developed into schistosity. The mashing and cementation of 

 the original sandstone may take place at the same time, or the 

 quartzite may be produced by the latter process and subsequently 

 converted into schist by compression. 



Mica Schist is principally composed of quartz, muscovite, and 

 biotite, with more or less felspar. By an increase in the quantity 

 of felspar present, and a coarser foliation, they grade into gneiss, 

 and by an increase of quartz they may pass into quartzite and thence 

 to sandstone. Through the phyllites mica schists are connected 

 with the slates, and in another direction, by increase of lime they 

 pass into argillaceous limestones. Mica schists are very largely 

 exposed in New England and southward along the eastern flank 

 of the Appalachian Mountain system. 



Hornblende Schist is a foliated rock, consisting of hornblende 

 with a varying proportion of felspar and less quartz. The horn- 

 blende schists are, for the most part, derived from the dynamic 

 metamorphism of various basic igneous rocks, the augite being 

 readily converted into hornblende by crushing, but in rare instances 

 they are believed to have had a sedimentary origin. The horn- 



