BY W. H. TWELVETTtEES AND W. F. PETTERD. 17 



greatest elasticity in the pyroxene. This is an important 

 optical test for distinguishing soda pyroxene from normal 

 angite. The groundmass is a remarkable feature of the 

 rock. Small laths of sanidine, often with fluxional 

 arrangement, form a ground-work, with which are 

 entangled pale green rods and needles of the soda- 

 pyroxene aegirine. These are slightly pleochroic, and 

 might be mistaken for augite, but that they uniformly 

 extinguish nearly parallel to their long axis, which direc- 

 tion of extinction the quartz wedge shows to be that of 

 the a axis of optical elasticity. The rods are sometimes 

 acicular at one or both extremities, sometimes curved. 

 They occasionally attach themselves end on like a fringe 

 to the borders of crystals of augite. They call to mind 

 the aegirine needles in the aegirine- (formerly called 

 acmite-) trachyte of the Kiihlsbrunnen in the Sieben- 

 gebirge. 



In one of our slides is an equilateral hexagonal section 

 of a small water clear mineral in the groundmass, greatly 

 resembling a section of nepheline ; l)ut it is not jjerfectly 

 isotropic between crossed nicols, and we have failed to 

 obtain a dark cross in convergent polarised light. It has 

 j)eripheral and central inclusions of colourless to pale 

 green pyroxenic microlites. It has no border such as is 

 common in noseans. If it is nepheline, it would remove 

 our rock from the trachyte to the phonolites ; for the 

 present we must leave the determination doubtful. In 

 the groundmass there is a good deal of isotropic zeolitic 

 matter, apparently of the nature of analcime. 



Another variety of the same rock is found on Mount 

 Mary, just above the mine. There it is a compact green 

 rock, often laminated, strongly resembling a metamorphic 

 rock. A few isolated scattered crystals of sanidine occur 

 in it, together with an occasional small black garnet. 

 Under the microscope we see that the green colour is due 

 to the felted netw^ork of aegirine needles, and that the 

 rock is essentially identical with the one just described, 

 only with the porphyritic felspars reduced to a minimum. 

 The garnet is the usual melanite variety, brown in thin 

 section. This rock contains pyrites. 



Aegirine Trachyte. 



Sp. gr. 2-61. 



Occurs on Mount Mary, just above the mine. 



Miner a I constituents. 



Sanidine, augite, melanite, titan ite, aegirine, biotite^ 

 apatite. 



