72 Proceedings of the Royal Society of Victoria. 



occurs ill two distinct forms, of allogenic and authigenic origin. 

 The allogenic felspar is present as large phenocrysts which have 

 suffered much erosion ; around them the lath-like felspars of 

 authigenic origin are arranged in flow-structure. In natural light 

 the allogenic felspar is seen as a clear and colourless material, 

 containing inclusions of a light brown colour, and magnetite. 

 As these inclusions are isotropic in character, it would appear 

 that during the intra-telluric stage in the history of the crystal 

 the solvent action of the magma produced a honeycomb structure 

 in the felspar, and that the cavities so produced were filled with 

 basalt glass, from which the magnetite has subsequently 

 crystallised out In one phenocryst both the glass inclusions 

 and the magnetite appear arranged in lines along the twinning 

 planes. The phenocrysts show twinning according to both the 

 albite and pericline laws; but no sections are present in the 

 slide from which measurements could be taken to determine the 

 nature of the felspar. The authigenic felspar occurs in sections 

 of short, broad prisms and in lath-like forms, which are sometimes 

 frayed out at one end. From measurements taken by us, this 

 felspar is probably labradorite. Augite is moderately represented 

 in the slide; it occurs mainly in allotriomorphic crystals, which 

 often form nests or clusters. These clusters give a striking 

 appearance to the slide. The masses of augite are destitute of 

 crystal boundaries ; their composite character is shown by the 

 fact that, under polarised light, difl'erent parts of the aggregation 

 do not obscure simultaneously. These clusters contain numerous, 

 somewhat large, grains of magnetite. In this slide there is a 

 good example of twinned augite showing the hour-glass structure. 

 Olivine is fairly aliundant as hypidiomorphic crystals and as 

 grains wedged in between the lath-like felspars; it is, for the 

 most part, decomposed into a light green chloritic or sei'pentinous 

 material. The magnetite and calcite do not call for any special 

 remark. The order of crystallisation of the constituent minerals 

 appears to be allogenic felspar, magnetite, augite and olivine, 

 authigenic felspar and secondary calcite. 



Slide No. 2 represents the typical basalt of the tongue-like 

 mass lying between the Pyrete and Goodman's Creeks. It has been 

 prepared from a specimen taken about half a mile north of " the 

 Neck." It has a tine-grained matrix, studded with porphyritic 



