GEOLOGY AND PETROLOGY OP ENOGGERA GRANITE. 145 



the present time, but future mining operations in the area 

 may furnish data on this matter. 



To return to a consideration of the Petrology of the 

 Rhyolitie Intrusives. The naming of the fine-grained acid 

 intrusives was a matter to which the author gave consider- 

 able attention. In spite of their definitel^^ intrusive 

 character and the fact that many of them have been con- 

 siderably altered by the addition of secondary silica, they 

 appear to be petrologically more closely related to the 

 Ehyolites than to any other rock group. I\lr. Ball, in 

 referring to rocks of this series, uses the terms ''felsite'' 

 and "felsitic," which certainly reflect the mode of 

 occurrence better than the author's term "Rhyolite" unless 

 ■one is careful to qualify it by the word intrusive, but the 

 absence of felsitic textures in the rocks examined under the 

 microscope and, further, the fact that they can be closely 

 correlated with the intrusive "Rhyolites" (so-called by 

 Andrews and Saint-Smith) of New England {see Table II.). 

 has led the author to retain the term "Ehyolitie 

 Intrusives. ' ' 



{!). 12) 171. — Dyke near junction of creeks in Portion 

 681, Parish of Indooroopilly. {See INIicrophoto- 

 graph Plate III., No. 9.) 



Megascopic. — A fine-grained greyish rock showing very 

 small yellowish-brown felspars and small patches of pyrites 

 set in a fine-grained grey base. A vein of ciuartz from 

 1-2 mm. across is to be seen traversing the specimen. 



Microscopic. — Very fine-grained holocrystalline rock, 

 with small vaguely defined decomposed phenocrysts of 

 felspar (orthoelase :) set in a ground mass made up 

 entirely of Quart z and Orthoelase. Only part of the forme i' 

 mineral seems to be primary, as there is considerable 

 evidence of secondary silicifieation. Pyrites occurs as fresh 

 individual crystals of very small size and as larger 

 aggregates. 



Name. — Intrusive Rhyolite. 



{D.6i) 115. 



This specimen is of particular interest from the 

 economic point of view. It was obtained by Mr. L. C. 

 Ball at a depth of 130 feet in the main vertical shaft of 



