THE GEOLOGY OF MITTAGONG. Boa 
Magnetite oe a8 St 45 a 
Apatite 
Analcime 
Felspars are of two orders. The large phenocrysts are 
of square or oblong section all showing polysynthetic twin- 
ning, some after albite and pericline laws, others albite 
and carlsbad type. Many crystals contain small patches 
of kaolin and other indefinite decomposition products. The 
felspars of the groundmass occur in laths, and are most 
usually multiple twin forms. By Becke’s method their 
R.I. is higher than Canada balsam, and hence they are 
basic plagioclase. | 
Augite is mainly in form of large rounded grains, some 
however showing a well defined octagonal crystal form. 
Colour is a sage-brown with very feeble pleochroism. The 
crystals are undecomposed but show characteristic cracks; 
inclusions of magnetite are not uncommon. 
Magnetite as small black crystals of square or triangular 
shape. It is also disseminated through portions of the 
slide in form of dust-like particles and clustered into small 
aggregates. 
Apatite is very abundant in form of colourless prisms 
and hexagonal cross sections. The crystals are rather 
larger than usual in dolerites. 
Irregular patches of clear colourless isotropic material 
of fairly large size with low R. index are probably analcime. 
Order of Consolidation. 
Felspar (Phenocrysts) | 
magnetite, apatite 
| 
| augite 
| felspar 
Name.—The rock is a DOLERITE very much like that figured 
in Harker’s Petrology (fig. 42). 
