296 



granulite has reference rather to texture than to composition. The main 

 mass of the normal granulite is an aggregate of small grains of quartz and 

 felspar. Under crossed nicols it presents the appearance of a mosaic. The 

 adjective granulitic is frequently applied to such a mosaic. Garnet is 

 commonly present in the Saxon granulites. The normal granulite (weiss- 

 stein) which is an acid rock is associated with the so-called trap-granulite 

 (pyroxene granulite), a basic rock, essentially composed of pyroxene 

 (rhombic and monoclinic) plagioclase and iron-ore. 



Halleflmta. This is a Swedish term. Typical hallefiinta is a hard 

 compact rock with a banded structure and having approximately the 

 composition of felsite. Rocks of diverse origin are grouped under this 

 term. Those associated with the normal granulite of Saxony are held by 

 LEHMANN to be of metamorphic origin. 



The following table will assist the reader in understanding the sense in 

 which the most important terms are used in the present work : 



ACID ROCKS. 



Volcanic. Liparite, Rhyolite, Vitrophyre, Obsidian, Pitchstone and many 

 felsites. 



Intermediate. Certain felsites and / 



f , ., , . > Granophyre. 



felsite-porphyries. i ,,. J . 



\ Micro-granite. 



Plutonic. Certain Granites and Gneisses. 



METAMORPHIC ACID ROCKS. 

 Volcan ic. Certain felsites (devitrified rhyolites, obsidians and pitchstones). 



Certain porphyroids and sericitic schists (metamorphosed 



liparites and liparite-tutf's.) 



Intermediate. Certain porphyroids (metamorphosed quartz felsites). 

 Plutonic. Certain gneisses and mica-schists (metamorphosed granites); also 



such rocks as greisen and luxullianite. 



SuB-AciD ROCKS. 

 Volcanic. Trachyte. 



Intermediate. Ortho-felsite or Orthophyre. 



Plutonic. Hornblende-syenite (syenite proper), Augite-syenite, Mica- 

 syenite. 



THE CONSTITUENTS or ROCKS OF GROUP C. 



Monoclinic felspar. In the plutonic and intermediate rocks this felspar 

 is generally more or less turbid ; in the volcanic rocks it is frequently 

 glassy. The term orthoclase is used to cover both varieties; the term 

 sanidine is limited to the glassy variety. 



When two generations of orthoclase are present the individuals of the 

 first generation, except in so far as they have been broken by mechanical 

 movements either during or subsequent to consolidation, are idiomorphic. 

 The most important forms are P (001) ; M (010); I (110); x (101), and // (201). 



