BY W. G. WOOLXOUGH. 521 



cavities makes the determination of specific gravity in mass 

 valueless as a criterion of chemical composition, and their small 

 size would necessitate very fine crushing to eliminate the error. 

 The specific gravity in mass is 2-58. 



Jlic7'oscopic characters.— \n thin section the rock is obviously 

 rendered strongly porphyritic by felspar, augite and hypersthene. 

 The base possesses a distinct hyalopilitic texture. The amount of 

 glass is relatively large and is quite colourless. Through it are 

 scattered such large numbers of microlites as to give it the 

 appearance of a grey "felt" under low powers. Under mode- 

 rately high powers these microlites are seen to consist of (1) very 

 numerous sti'aight and curved hair-like microlites undeterminable 

 under a magnification of 750 diameters; (2) plentiful rod-shaped 

 augites; and (3) small felspar laths. 



The augite rods, with very sharpl}" defined boundaries, run ujd 

 to about 0*026 mm. x 0*006 mm. as a rule, a few odd ones being 

 about twice as much in each direction. In colour they are 

 greenish-yellow, and have extinction angles up to 45*^. The 

 double refraction is quite noticeable. 



The felspar microlites are of about the same order of size as the 

 augites, but their boundaries are by no means so sharply defined 

 on account of their much lower refractive index. This is less 

 than that of Canada balsam. The extinction angle, so far as 

 it can be measured, is within a few degrees of straight. These 

 facts indicate that the felspar is oligoclase. 



Grains and crystals of magnetite are moderately abundant. 

 The vesicular character, so obvious in hand specimen, is much 

 less marked in section. The irregular shape of the cavities makes 

 them look like accidental holes in the section. The film of 

 material immediately in contact with the walls is almost opaque, 

 and shows white by reflected light. By transmitted light it is 

 very dark brown on account of its opacity. It is apparently 

 fibrous and isotropic. Here and there there are other zeolitic 

 materials present in small quantities. Parts of this substance 

 have a refractive index almost the same as that of Canada balsam, 

 and are perfectly isotropic; other parts have a refractive index 



