

MICKOSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 203> 



scopic dust rendering the glassy portions of the rock almost opaque^ 

 and thirdly, in rods that are often bent and broken. A drawing 

 from a microphotograph of this beautiful rock is given on Plate 

 xxi., fig. 1. The figure is fairly typical of the basalts found in so- 

 many localities around the town of Orange. In fact I have slides 

 cut from basalts collected from places fifteen and twenty miles 

 apart, showing the structure figured so closely as not to be dis- 

 tinguishable. The structure reminds one too, of many of the 

 Tertiary basalts of Europe and the United States of America* 

 Immediately around Orange, basalts structurally similar are found 

 at Lucknow, Forest Reefs, the Race Course Paddock, and at the 

 the Corporation Quarries. 



Slice 13. — From a basalt flow, about half way between the town 

 of Orange and Mt. Canoblas, New South Wales. The general 

 structure of this slice is best made out with a two inch objective. 

 When the slice is placed transversely on the stage of the micro- 

 scope, great streams of felspar are observable up and down the 

 field. Scattered through this flow structure, are aggregations of 

 magnetite and augite granules. These are mostly confined to the 

 "eyes" formed by the wavy outlines of the streaming felspars. 

 In polarized light, a great deal of glassy matter can be detected 

 in some of the olivine granules. There are no idiomorpic minerals, 

 other than magnetite and felspar. All these basalts, collected 

 around Orange had a common source in the Canoblas. The whole 

 series of volcanic material is made up of several distinct flows, and 

 at present there is no evidence to show that any considerable time 

 intervened between the successive outbursts. I have not been 

 able to discover any interbedded material that would point to a 

 period of rest, but the material on hand, collected during my 

 own few short visits to the mountain, is hardly sufficient to settle 

 anything on this point. A microscopic examination of the various 

 lavas shows that the volcano poured out acidic and basic, as well 

 as rocks of intermediate composition, which latter will probably 

 be classed with the andesites. The great bulk of the acidic and 

 intermediate rocks have disappeared through denudation, but 



