Andrew. — On the Clarendon Phosphate-deposits. 459 



Basalt. 



Occurrence. — The basalt sheet overlies the calcareous series 

 and forms the cap of the higher hills. It is practically con- 

 tinuous on the west side of the main road, running down from 

 the Trig. Q to the lower levels. Outliers occur at Cemetery 

 Hill, Kapiti, and behind Strain's farm. Beneath Trig. Q it 

 must be at least 300 ft. in thickness. 



Character. — The basalt is finely grained, with a sharp clean 

 fracture. At Cemetery Hill it has a columnar structure with 

 rude hexagonal jointing. Under the microscope the rock from 

 Cemetery Hill is seen to consist of broad plates of a basic plagio- 

 clase wrapping round and enclosing crystals of augite, anomite, 

 magnetite, ohvine, and serpentine. The plagioclase, which forms 

 the greater part of the groundmass, is a very basic variety, 

 probably bytounite. Augite is the most common coloured con- 

 stituent of the rock, and has the usual characters. Anomite, 

 a rather unusual variety of biotite, occurs here in irregular 

 flakes with intense pleochroism. Magnetite is very abundant. 

 Olivine is not at all common, and neither is its decomposition 

 product serpentine. I looked carefully for apatite in every 

 slide, but was unable to see it ; neither did micro-chemical 

 tests reveal the presence of phosphorus. 



Nearly thirty slides were examined from various parts of 

 the district to see if there was any variation in the basalt. Dif- 

 ferences were detected, but these were not of great importance. 

 At Stony Knob magnetite is not so abundant as at Cemetery 

 Hill, and occurs chiefly as specks and granules, not crystals ; 

 anomite is less abundant ; the broad plates of plagioclase are 

 not so conspicuous ; while well-defined feldspar-laths with albite 

 twinning become common. At Trig. Q plagioclase is abun- 

 dant as minute stout lath-shaped albite twins ; anomite is 

 entirely absent. Near Williamson's magnetite becomes very 

 abundant ; olivine becomes common, while the plagioclase 

 is present only as albite twins , anomite is entirely absent. 

 At Strain's and near J. Gray's farm the rock is much altered 

 by weathering : the feldspars are abundant as stout crystals ; 

 the augite is mostly decomposed ; the olivine is changed to 

 serpentine ; and the whole mass of the rock is stained a 

 yellowish - red by iron-oxide. At Table Hill, however, four 

 miles south - west from Millburn, the structure changes to 

 that of a fairly coarse dolerite, with labradorite as the most 

 prominent phenocryst ; augite is here confined to the ground - 

 mass, which contains no glass ; magnetite and olivine are 

 common. 



