190 REV. J. MILNE CURE AN. 



while others, as those of Young, Nymagee, north of Armidale, 

 ^ind Araluen are pseudo-porphyritic, that is, large crystals are 

 developed in an already coarsely crystalline mass. 



Amongst the basalts we have many interesting varieties of 

 structure to record. At Bathurst, there is a long line of hills 

 capped with basalt that flowed down from Swatchfield.* In 

 macroscopic structure it resembles many well known basalts of 

 Hie Old World. It is prismatic in the field, and under the 

 microscope micro-porphyritic in structure with idiomorphic augites 

 and olivines. The micro-slices of this rock show beautiful examples 

 of a streaming structure of the felspars. 



Another basalt extensively developed further west about Orange 

 Oarcoar, and Cargo, is of an entirely different microscopic struc- 

 ture. Here we have the felspars forming the only idiomorphic 

 constituents, with the olivines and glassy matter wedged in 

 between. Still further west, in the same colony, there is yet 

 another type of basalt. It is found in many localities about Dubbo 

 but chiefly to the east of that town in a huge stream running 

 north and south. In this rock the olivines are all altered to 

 various secondary minerals. The macroscopic structure ranges 

 from the average basalt to a dolerite, and when the olivine has 

 completely changed, the rock might be called a diabasic dolerite. 



On the northern slopes of the Canoblas, there is an interesting 

 porphyritic basalt. The ground-mass or paste is an exceedingly 

 fine micro-crystalline aggregate, chiefly of plagioclase, magnetite 

 ^tnd unindividualized glassy matter. In this are set porphyritic 

 crystals of oligoclase. 



Basalts showing a decided ophitic structure are represented 

 from Dubbo, and from many of the New England basaltic plateaux. 

 Fluidal movements, while in a molten condition, are indicated in 

 a great number of the slices by the arrangement of the felspars, 

 and by the presence of fractured crystals. It is very noteworthy 

 that in the felspars of all our Australian Tertiary eruptive rocks, 



* Wilkinson, Ann. Rept. Dept. Mines N.S.Wales for 1878, [1879.], p. 238. 



