84 



Miscroscop'tc Characters. 



Crystallinity hypocrystalline, with a hyalopilitic ground- 

 mass containing porphyiitic crystals of olivine and augite. 

 There is a notable quantity of a dark-brown glass present, 

 which, under a high-power objective, is found to contain a 

 considerable quantity of magnetit/e dust. One of the most 

 interesting features about the glass is that it contains deli- 

 cately-branched rods or skeleton crystals of magnetite which 

 are probably of the second generation. 



The microlitic components of the base are chiefly plagio- 

 clase felsjoar, and grains of augite and olivine, with a more 

 or less idiomorphic outline developed in the former. 



The felspar microlites make up the greater part of the 

 groundmass, and in places show a slight fluxional arrange- 

 ment. In crossed nicols they show twinning, mostly after 

 the Albite type, although the Carlsbad type is not uncom- 

 mon. 



The refractive index was found to be greater than Can- 

 ada balsam on the edge of the section where it was measured 

 b}'- Becke's bright-line method. Symmetrical extinctions up 

 to 36° were obtained, the greater number of extinctions be- 

 ing between 21° and 36°, thus indicating labradorite. 



Most of the microlites contain inclusions, usually gase- 

 ous or liquid, and less frequently small crystals of apatite, 

 but they do not show any zonal arrangement. 



The augite components of the base are in elongated 

 prisms with a yellowish -green colour. The high extinction 

 angle and refractive index are characteristic of augite. There 

 is also a fairly good prismatic cleavage developed. Besides 

 this type of crystal there are stumpy crystals and grains of 

 the same mineral showing traces of the (100), (010), and 

 (111). The grains frequently show two good cleavages in- 

 tersecting in angles of about 90°. 



The olivine of the base is present in small grains, with 

 irregular cracking. It is" not so plentiful as the augite, but 

 can be differentiated from augite by its strong double refrac- 

 tion. 



The phenocrysts of the rock are olivine and augite, the 

 former being the more abundant, and in the majority of cases 

 larger than the augite phenocrysts. The olivine phenocrysts 

 have been partially absorbed by the magma, although some 

 possess an idiomorphic outline. Decomposition has not taken 

 place to any great extent, which is proved by the fact that 

 very little serpentine is found in the cracks. They are about 

 2 mm. to 3 mm. in length, and are perfectly colourless. A 

 fair cleavage is shown paralled to the (001) in some pheno- 



