BY W. G. WOOLNOUGH. 795 



at Devil's Hole(p.792). Against this idea is the fact that no 

 trace of associated basalt-flows has been found. 



Petrology of the Eruptive and Metamorphic Rotks. 



Grano-diorite: Glenrock Falls. (Plate Ixix., fig. 1). 



The rock is phanero-crystalline, with an apparent average 

 grain-size of about 2mm. Body-colour light grey, due to mixture 

 of black, grey, and white minerals. Structure, granitic. 

 Minerals distinguishable : — greenish-grey plagioclase, pinkish 

 orthoclase, clear quartz, plentiful black hornblende (and hyper- 

 sthene, which is not distinguishable from the hornblende macro- 

 scopically) and small quantities of shining biotite. 



Under the microscope we have : — Texture, liolocrystalline, 

 phaneric, average grain-size 0-75 mm.; hypidiomorphic granular. 

 Minerals present, and approximate percentages — plagioclase 35, 

 quartz 25, orthoclase 15, diopside and uralite 8, hypersthene and 

 bastite 5, hornblende 5, biotite 5; remainder — magnetite, zircon,, 

 apatite, and epidote. 



Plagioclase is in strongly idiomorphic crystals up to 2 mm. by 

 1-5 mm., strongly tabular in habit, and very beautifully zoned. 

 Albite-twinning is universal; many individuals show pericline 

 and Carlsbad twinning also. The inner zones consist of basic 

 andesine, about Ab^An.3, giving symmetrical extinctions in the 

 brachypinacoid zone up to 23^*, with a difference between the 

 halves of a Carlsbad twin amounting to 5*^. The outer zones 

 consist of oligoclase, andesine about AbgAno, with symmetrical 

 extinctions of 8^ These crystals are considerably decomposed, 

 particularly in their central portions, to interlacing fibres of 

 sericitic mica. 



The quartz is completely allotriomorphic. It is entirely free 

 from strain-effects. No features of special interest are to be 

 recognised. 



Orthoclase is in completely allotriomorphic grains, untwinned 

 and almost completely kaolinized. 



Diopside is in subidiomorphic crystals of stout prismatic form,, 

 mostly intergrown with primary hornblende. The two ar& 



