BY W. H. BRYAN, B.SC. 159 



These later "' Acid " granites of New England have 

 been considered by different observers to be Perino-Carbon- 

 iferous,^' ^ ' Early Mesozoic."^' ^ and Mesozoic^ in age. 

 A consideration of these opinions and especially the more 

 recent ones seems to point to late Permo-Carboniferous 

 as being the most probable age of intrusion. 



Assuming then that the Enoggera granites and the 

 " acid " granites are related as regards time of intrusion, 

 this resvilt gives weight to the decision, already formed on 

 purely local evidence, that the granites are late Permo-Car- 

 boniferous, which may then be supposed to approximate 

 the truth. 



vi. The Rhyolitic Intrisives. 



This is the name given to an extensive series of intrusives 

 which occurs throughout the area, the great majority of 

 them however, lying to the south of the main granite 

 outcroj). The rocks which come under this head vary 

 considerably, but are as a rule light in colour, the commonest 

 shades being white, light brown and grey. They include 

 intrusive rh^'olites, felsites and fine-grained granojhyric 

 rocks. They are frequently porphyritic — the phenccrysts 

 generally being orthociase — but in no case is this structure 

 pronounced. Well marked fluxion structure is very com- 

 mon, especially in the more acid varieties. A feature which 

 is characteristic of the group is its mode of weathering. 

 Almost without exception the rocks weather into very 

 angular fragments with sharp edges. The planes along 

 which the rocks break intersect each other at all 

 .angles, the cracks generally being filled with limonite. 

 In this respect the group is very strongly differentiated 

 from the younger prophyries to be described later. A 

 common mode of alteration in these intrusives is caused 

 by the continuous passage of siliceous waters through them. 

 The rocks thus attacked gradually become more and more 



1. Fedeial Hand-book, B.A.A.S. 



2. Andrews in the Handbook nf K.S.W., issiied for B.A.A.S., p 



3. Came, Op. cit. (on maps). 



4. Andrews. Op. cit. p. 113. 



5. Came, Op. cit. 



