224 ON THE GEOLOGY AND PETROGRAPHY OF BATHUKST, N.S.W., 



seated rocks of the class in the same area. May we not in these 

 cases explain the phenomena in the way suggested by M. Michel- 

 Levy by the consolidation having taken place at two different 

 periods? It is not difficult to imagine conditions which would 

 bring about such a result. If, for example, a mass of igneous 

 materials were in a liquid state at a great depth from the surface, 

 the conditions might be favourable to the separation of a felspar 

 of a given composition from the magma. The continued abstrac- 

 tion of certain elements from the base would alter the composition 

 of the surrounding magma, and this would modify slightly the 

 conditions causing the successively formed zones 'of the crystal to 

 vary slightly in composition. But if a fissure were formed above 

 such a molten mass, then the pressure upon it would be greatly 

 and suddenly relieved, even though no actual movement occurred 

 in the deeper-seated portion. Under the entirely new conditions 

 thus originated, the magma surrounding the zoned crystals already 

 formed might be induced to crystallise in a totally different manner, 

 the order of the separation of the minerals and the forms and 

 relations of their several crystals being determined by these new 

 conditions." 



As some of the minerals in the basalt present features worth 

 noticing, I may refer to the characters they present when seen 

 under the microscope. 



Olivine. — The porphyritic crystals of olivine are so abundant 

 that, with very few slides, sections may be found in various zones 

 sufficient to study its leading optical properties. I have noticed 

 sections close to basal planes, and sections approximately parallel 

 to the macropinacoid, so as to show an interference figure in con- 

 vergent polarized light. Sections roughly showing the form of an 

 elongated hexagon are plentiful. The peculiar ground glass 

 surfaces, due to its high refractive index, are very pronounced. 

 In fairly thin slices the mineral shows a very faint yellow-brown 

 colour. But the most remarkable feature in the olivine is the 

 fact that it is the first mineral in the rock to fall a victim to 

 alteration. Every large crystal shows serpentinous lines of 



