MICKOSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 187" 



Around Sunny Corner, fine examples of porphyritic quartz, 

 felsites occur. The quartz can be seen from small grains, just 

 visible to the unaided eye, up to blebs 5 mm. in diameter. The 

 best examples occur on the hill just above the township. A well 

 developed flow structure is often noticeable, the lines of fluxion 

 parting around the porphyritic ingredients and meeting again. 



At Cargo, there are abundant outcrops of felsitic rock. At 

 the Ironclad Mine it is found forming the " hanging wall " of an 

 auriferous lode. The felsite is evidently an intrusive rock. It 

 contains porphyritic crystals and blebs of quartz. The quartzes 

 are drawn out into long strings, as if the whole mass moved slowly 

 in one direction under great pressure. 



At the Railway Station, Carcoar, near the tunnel on the 

 Hospital Road, and at the Glen, near the same town, compact 

 felsites are intruded into Silurian rocks. Under the microscope 

 a sphserulitic and sub-sphseruHtic structure is apparent in many 

 of the slices. 



To the south of Dubbo, on the edges of an extensive granitic 

 area, there are acidic rocks which must be placed under the head- 

 ing of altered rhyolites. They are all flinty in character, and are 

 of Pre-Tertiary age. They carry microscopical quartz crystals 

 that show signs of considerable corrosion, and, in some instances, 

 obscure traces of felspars remain. They have all the characters 

 of devitrified glassy rhyolites. These are the only rocks in which 

 I have found the interesting structures known as axiolitic. The 

 aggregations in the felsitic base are arranged in a longitudinal 

 manner about a wavy axis. 



There is a very considerable development of acidic lava flows. 

 on the Lachlan River above Cowra. They are seen to the best 

 advantage near the junction of that river and the Rocky Bridge 

 Creek. These rocks are associated with basic basalts in the same- 

 way as in modern volcanoes, acidic and basic materials flow from 

 the same centres. The cavities in this lava are filled with 

 amorphous silica and opal. The lath-shaped felspars that form the 



