293 



the rock of the Plauenschen Grande near Dresden by WEHNER. The 

 Egyptian rock is a hornblende-biotite-granite : the Saxon rock is essentially 

 composed of orthoclase and hornblende. Plagioclase, quartz, biotite and 

 sphene occur as accessories. The term is now generally applied to ortho- 

 clase-bearing rocks in which quartz is present only in very small quantity 

 and may be absent altogether. In this sense the syenites are rocks of 

 intermediate composition so far as silica percentage is concerned, but they 

 differ from the normal intermediate rocks in having more potash than soda- 

 The structure of all syenites is granitic. Mineralogically syenite is the 

 plutonic representative of trachyte and, like trachyte, it does not appear to 

 be a very widely distributed rock. The term has been somewhat loosely 

 used in this country. It is doubtful whether any rocks answering to 

 WERNER'S type have been recognized. 



Augite-syenite. This term was introduced by VON RATH for certain 

 rocks of granitic texture which consist of orthoclase and augite, together 

 with small quantities of plagioclase, iron-ores and sphene. The type-rocks 

 occur near Predazzo in the Tyrol. They are associated with plagioclase- 

 augite rocks of similar texture (diabase of VON RATH, gabbro of most 

 authors) into which they pass by regular gradation. The term mon~onil<' 

 is sometimes used instead of augite-syenite. 



Mica-syenite. This term is applied to granitic rocks essentially com- 

 posed of black mica and orthoclase. Mica-syenites are comparatively un- 

 important so far as distribution is concerned. 



Enstatite-syenite. As we have hornblende-syenites (syenites proper) 

 augite-syenites and mica-syenites we should naturally expect, from analogy 

 with other rock-groups, to find a rock answering to this term. Mr. 

 WILLIAMS (1) has recently described such a rock under the term Norite. It 

 occurs in the Cortlandt series and consists of orthoclase, andesine and 

 hypersthene, together with small quantities of biotite, apatite and iron-ore. 



Obsidian find Pitchstone. These terms are applied to the glass}* 

 forms of the acid magma. 



So far we have been referring more especially to the terminology 

 of comparatively unaltered rocks. Many felsites and feLsifce porphyries 

 (felsophyres) are, it is true, devitritied obsidians, pitchstones and vi trophy i*es ; 

 but we are at present unfortunately unable to separate these in all cases 

 from the corresponding rocks in which the double-refraction of the felsitic 

 matter is a consequence of the actions accompanying solidification 

 (primary devitrification). The various agencies of metamorphism produce 

 changes in the rocks of the present group as in those of every other group, 

 and some special names have been applied to special types. In in;my 

 cases the names were introduced before the true nature of the rocks 

 was recognized, so that the terminology of the metamorphic rocks is more 

 than usually defective. Again, it may be regarded as certain that similar 



(1) A.J.S. Vol. XXXIII., 1877, p. 138. 



