RELATION OF SYENITE TO SOME SLATES. 

 Analyses comparing Syenite witli Cambrian Slates. 



225 



Diorite. 



Dioiites are essentially combinations of plagioclase and horn 

 blende, but in some localities orthoclase, apatite, and magnetite or 

 titanic iron appear as subordinate constituents ; while titanite, mag- 

 nesia-mica, and pyrites are sometimes present as accessories. The 

 plagioclase was formed first, and occurs in grains or small rods ; but 

 when the rock is porphyritic, these crystals are tabular. In the 

 diorites of Guernsey, the Yosges, and many other localities, the twins 

 cross at a right angle, or an angle a degree or two less. Fluid inclu- 

 sions are rare. The mineral is sometimes so clear as to resemble quartz, 

 but is generally opaque ; and when the plagioclase is decomposed, cal- 

 cite is often developed. The orthoclase in diorite is like the orthoclase 

 of granite. It is chiefly found in quartz-diorite. Hornblende occurs 

 in films or short columns. In the deeper-seated North-German rocks 

 the crystals are broad and large. On the western slope of the Vosges 

 the needle-diorite occurs, in which hornblende forms small slender 

 needles among the rod like crystals of felspar. Its colour is com- 

 monly green, but sometimes brown, as in the quartz diorites of the 

 Odenwald and Vosges. It incloses magnetite, apatite, and sometimes 

 plagioclase and titanite. Where mica is present its crystals penetrate 



I 



1 Clifton Ward. Q. J. G. S., vol. xxxii. pp. 5, 22. 



VOL. I. 



