Grange. — Geology of Green Island Coalfield. 



169 



DOLERITE. 



The word " dolerite " is used in the sense in which Marshall (1906, 

 p. 410) employs it — namely, " as a term covering all the types of coarse basic 

 rocks, irrespective of age, if they are of effusive character." Dolerite caps 

 Kaikorai and Abbott's Hills. The rock weathers to sphenoids, leaving in 

 the spaces white bands of sepiolite and reddish-black bands of iron oxide. 

 The "dolerite can easily be distinguished in the field, since the augite pro- 

 jects on the weathered surface. In fresh samples the augite, olivine, and 

 feldspar can be distinguished. Steam-jiores are abundant, and often filled 

 with amygdules of secondary calcite. 



Phenocrysts. — Labradorite (1-05 mm. X 0-3 mm.) is not plentiful. Augite 

 occurs in large idiomorphic crystals (3-1 mm. X 1*25 mm.), and is of a 

 violet colour showing strong pleochroism. Twinning is frequent. One 

 crystal has zonal banding, successive layers differing in tint indicating 

 differences in the titanium content ; another shows hour-glass structure. 

 The augite commonly encloses olivine in a poecilitic fashion. Olivine 

 (2*1 mm. X 048 mm.) is sharply idiomorphic, and is altered to serpentine 

 along practice-lines, but not to the same extent as in the basalts. Of the 

 olivine and augite, the former was the first to crystallize. 



Matrix. - - The matrix is coarse, being made up of feldspar laths, 

 (labradorite), augite, magnetite, and a little olivine. 



There is little distinction between the groundmass and the phenocrysts 

 The following analysis shows the composition of the rock : — 



Silica (Si0 2 ) 



Alumina (A1 2 3 ) . . 



Ferric oxide (Fe 2 3 ) 



Ferrous oxide (FeO) 



Lime (CaO) 



Magnesia (MgO) . . 



Potash (K,0) 



Soda (Na 2 6) 



Combined water (H 2 0) 



100-22 

 Trachydolerite. 



The trachydolerite of this area is found on the northern slope of Abbott's 

 Hill, whence it stretches for many miles to the north towards Flagstaff. 

 A contact with the other lava-flows could not be found. Trachydolerite in 

 the Dunedin district occurs both as a hypabyssal and as an effusive rock, 

 with no distinctive character that would serve to discriminate them under 

 the microscope. The wide extent of the rock above Abbotsford suggests 

 that it is a lava-flow. In no place in Dunedin is the trachydolerite inter- 

 calated Math other lavas (Marshall, 1906, p. 407). It probably followed 

 the eruption of the dolerite. 



In hand-specimens the rock is readily distinguished by the large crystals 

 of feldspar that project on the weathered surface. Microscopically it shows 

 the following : — 



Phenocrysts. — Large idiomorphic crystals of oligoclase (3-2 mm. X 5-8 mm.) 

 with extinction angle of 10°. It is twinned on the Carlsbad law. The 

 nepheline (0 - 2mm. x 0-8 mm.) is clear and often shows hexagonal outline. 

 Rarely light-blue sodalite occurs. A special feature of this mineral is the 

 great number of inclusions, most of which are arranged in lines with two 

 directions crossing at an angle of 60°. The inner part of the crystals 



