110 MESSES. J. K. DAKYNS AND J. J. H, TEALL ON THE 



Biotite. — There are two varieties of dark mica corresponding with 

 the two varieties of hornblende. Both are nearly uniaxial. The 

 biotite of the nltra-basic rocks occurs in large ragged plates, con- 

 taining inclusions of olivine. 



Pleochroism. 



S / '' N I pale brown. 



y (ng) rich reddish brown. 



The biotite of the intermediate and acid rocks occurs in small 

 plates, several of which are frequently grouped together. It rarely 

 shows definite crystallographic outlines. 



Pleochroism. 



i8(nl)}p^^^^'"'^^'''- 



y (ng) very dark brown, ahnost opaque. 



There is an absence of the red tinge which is characteristic of 

 the biotite of the basic rocks. Chlorite containing grains of epidote 

 arises as the result of the alteration of biotite. 



Plagioclase. — Amongst the ultra-basic rocks this mineral has been 

 observed only in the enstatite-augite-hornblende-biotite rock. It 

 plays the role of groundmass and serves to bind together the 

 ferro-magnesian constituents, so that they weather out as knots. A 

 section through one of these knots shows a few, apparently isolated 

 patches of twinned plagioclase, having the same optic orientation 

 over a large area. Notwithstanding the fact that the knots may 

 measure as much as three quarters of an inch in diameter, the actual 

 amount of felspar present is very small in comparison with that of 

 the ferro-magnesian constituents. In the granitic rocks the plagio- 

 clase shows a tendency to idiomorphism, and this is especially the 

 case when it is in contact with quartz or microcline. Eroad lath- 

 shaped sections, terminated by well-developed crystallographic 

 planes meeting in salient and re-entering angles, may occasionally 

 be observed where felspar is in contact with interstitial quartz. 



Orthodase and Microcline. — The potash-felspar occurs in crystals, 

 crystal fragments, and allotriomorphic grains. The large crystals 

 of the porphyritic granite are often Carlsbad twins having the forms 

 {llOj-, {001}, {010}, and {101}-. In thin section a zonal structure 

 is frequently seen. Sphene, iron ores, hornblende, biotite, and 

 oligoclase occur as inclusions in the porphyritic crystals, and iron 

 ores, sphene, and apatite may be seen as inclusions in the horn- 

 blende which is itself included. Microcline plays the role of 

 groundmass in some of the more acid rocks. 



Quartz. — This mineral is present in fairly large irregular patches 

 having uniform optic orientation (interstitial quartz), as aggregates 

 of allotriomorphic grains, and as a constituent of micro -pegmatite. 

 It contains minute cavities, some of which carry movable bubbles. 

 The inclusions are sometimes arranged in planes which may be 

 followed from one grain to another. Most of the cavities containing 



