410 Transactions. — Geology. 



II. Microscopical Examination. 



A microscopical examination of the rock shows it to be 

 composed of a holocrystalHne ground-mass, in which crystals 

 of feldspar, augite, olivine, and magnetite are porphyritically 

 distributed. 



Porpliyritic Minerals : Feldspar. — The feldspar is the most 

 important porphyritic mineral. It occurs in lath-shaped and 

 rounded forms up to ^j-vn. in length. The species of feldspar 

 as determined by the extinction of twin lamellse proved to be 

 anorthite, as the angle of extinction was noticed in several 

 cases to be 70°, and slightly over. However, the determination 

 was rendered inexact and difficult by the frequent occurrence 

 of undulose extinction. As no cleavage -flakes could be ob- 

 tained, no reliable determination could be made with con- 

 vergent light ; but the examination of sections which showed 

 no twinning, and would therefore probably be parallel to the 

 brachypinacoid, gave a revolving axial shadow and an optic 

 axis just outside the field of view. This would make it 

 bytownite or anorthite. So the conclusion to be come to is 

 that it is probably anorthite, a species of feldspar almost 

 typical of basic rock. 



Inclusions of magnetite are common, and on examination 

 with higher powers there appeared numerous small acicular 

 inclusions. These did not show straight extinction nor pleo- 

 chroism, so they cannot be apatite. Many of the crystals 

 show traces of alteration, and in some cases the crystals are 

 completely honeycombed. These alteration products are often 

 confined to the interior of the crystal, or arranged in zones, 

 while the outside is altogether free from them. 



The characteristic twinning is of the albite type, but cases 

 occur which show the cross-hatching due to twinning on both 

 the pericline and albite types, while one case showed three sets 

 of twin lamellae, one inclined at an angle of 20°, and the other 

 at an angle of 70°, to what seemed the ordinary twinning of the 

 crystal. This was parallel to the length of the crystal, and 

 could be traced passing through the others as if it had been of 

 the latest formation (PI. XLIX., fig. 1). This may show that 

 the state of polysynthetic twinning is the more stable state for 

 feldspars near the surface of the earth. The twin lamellae were 

 noticeable for the fact that they frequently died out, or occu- 

 pied but a small portion of the crystal. Several cases were 

 noticed in which an internal kernel was twinned, while the 

 outside portion was untwinned, or twinned in a different direc- 

 tion. The twin lamellae sometimes end abruptly at the edge 

 of the core, but occasionally are prolonged faintly to the edge 

 of the crystal. The most characteristic feature of the feldspar 

 is the curious evidence of crystal growth. In many cases the 



