The GcolcHju of Coimaidai. 71 



some hundreds of feet of rock to attain its present level. 

 Attention has been di-awn to the connection between the basalt 

 flow and the ancient river valley of the district. 



The physical relations of the basalt to the other formations do 

 not offer any points of particular interest. In places we have 

 found the sul))acent rock showing signs of metamorphism at its 

 junction with the basalt. At a point a few yards east of 

 Holloway's tunnel the basalt contains included pebbles of quartz 

 and quartzite similar to those found in the river drift ; at the 

 junction of the Coimaidai and Bullengarook Roads unrounded 

 fragments of sandstone are found in it, while at its southern 

 boundary it contains pieces of slate, etc. 



A large number of specimens of basalt from different parts of 

 the district have been prepared for microscopic examination. 

 The general result of the examination shows that the rock 

 may be classed as an olivine basalt with, however, a somewhat 

 varying character. The specimens sliced from the neighbour- 

 hood of Coimaidai and south of Coimaidai contain large eroded 

 crystals of felspar of allogenic origin, while olivine is somewhat 

 subsidiary. As we proceed towards the northern boundary of 

 our area the allogenic felspar disappears, and the rock becomes 

 very rich in olivine. In other slides a further disappearance of 

 authigenic felspar may be noticed until the rock assumes the 

 type of limburgite. Some of the specimens have been tested 

 for titaniferous iron, but with a negative result. 



In macroscopic structure the basalt shows the usual varia- 

 bility, passing by stages from a hard, dense rock into the 

 vesicular form. Magma-basalt, much weathered, was found near 

 the limbui'gite. 



Slide No. 1 is cut from basalt taken from the eastern edge of 

 the plateau, about a quarter of a mile due west from the township 

 of Coinjaidai. In appearance it is a dense rock, of bluish-black 

 colour, breaking with a somewhat hackly fracture. Porphyritic 

 crystals of felspar, olivine and secondary calcite may be seen by 

 the naked eye. Under the microscope the rock is seen to possess 

 a hyalopilitic structure ; there is a considerable amount of glass 

 or isotropic matter present, which has in natural light a faint 

 brown colour. The other minerals present are triclinic felspar, 

 augite, olivine, magnetite and secondary calcite. The felspar 



