MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 189' 



The Australian granites could be subdivided according as they 

 contain a preponderance of muscovite, biotite, or hornblende.. 

 But any such division would be arbitrary, as many of our granites. 

 contain two and some all three minerals in varying quantities in 

 different parts of the same mass. 



The actual structure of our granites reveals nothing new. So* 

 far as the writer is aware, no true granite has yet been met with 

 that gives internal evidence of a metamorphic origin. That many 

 of our granites are the result of pre-existing sediments being- 

 brought within a zone of fusion is a view which is receiving- 

 additional support each year. About Bathurst, New South Wales, 

 for instance, highly contorted, silurian slates are found resting on 

 granitic rocks. This granite sends veins and tongues of intrusive- 

 material into the overlying slates. The granite then is evidently 

 not the original floor on which the sediments were deposited. At 

 the same time no trace or vestige of this floor is anywhere to be- 

 seem The only explanation of this, by no means uncommon rela- 

 tion between clastic and holo-crystalline igneous rocks is that the 

 old floor was absorbed in the intrusion of the igneous mass. This 

 could have happened either by the ascent of the eruptive magma 

 or through the sediments being depressed into a zone of fusion.* 



A number of our Australian granites suffer from what is called 

 by Dolomieu " La maladie du Granite," and which he attributes 

 to the action of carbonic acid issuing from a subterranean source^ 

 A microscopic examination of these granites reveals the fact that 

 the " disease " is internal. The felspars are found milky and 

 opaque, with gas cavities and included matters. Cracks and 

 cavities show as pores in the felspars mineral, so that in reality 

 they are permeable to water. This is the secret of the decay. In 

 microscopic structure, we have granites ranging from porphyritic 

 to almost felsitic structure. Many are truly porphyritic granites 



7 



* For some very interesting matter on the origin of crystalline rocks 

 in this way, see a paper by Dr. A. C. Lawson Oh the Archaean Geology 

 of the Eegion north-west of Lake Superior — Etudes sur les Schistes 

 CristallinSjCongres Geologique International, 4me Session, Londres, 1888. 



