T5Q ENOGGERA GRANITE AND ALLIED INTRUSIVES. 



morphism resulting in the formation of gneissose textures, 

 in brecciation and the tearing off — by whatever means — 

 of fragments of the wall rock, occurs around the smaller 

 mass, which however does not seem to have influenced the 

 strike of the schists to any very noticeable extent. 



Mode of Intrusion — Of the three principal theories 

 at present put forward to account for the intrusion of granitic 

 magmas, the Marginal Assimilation theory, which is sup- 

 ported by some French geologists, suggests chemical energy 

 as the chief factor in the intrusion ; the Overhead Stoping 

 Hypothesis of R. A. Daly^- considers heat energy to be the 

 all-powerful agent, while the Laccolitic Theory, advanced 

 by Harker,^ Brogger* and many others, relies chiefly on 

 mechanical energy. 



If the above hypotheses be applied to the present 

 instance, it will be seen that the evidence in general supports 

 the Laccolitic method of intrusion for at least the large 

 Enoggera mass, as witness the following facts. 



The schists near the contact strike parallel to and dip 

 away from the granite. Here, obviously, great mechanical 

 energy must have been called into play, either in the pre- 

 paration by folding movements of a cavity or plane of 

 weakness in the schists into which the magma found its 

 way (forming the "' phacohte " of Harker^) or — and this 

 seems more probable, since the long axis of the intrusion 

 is not sympathetic with the strike of the schists^ — in the 

 actual lifting up of the schist cover by the invading magma 

 itself to form the typical laccolite. 



The contacts of schist and granite are always very 

 sharply defined. Traces of assimilation are quite lacking 

 and hence the Assimilation Hypothesis seems out of the 

 question . 



There is an almost complete absence of inclusions, 

 such few as are found showing no trace of absorption by 

 the magma. Further there is, too, an absence of what "Daly 

 terms the Zone of Apophyses. These facts militate against 



1. Mechanics of Igneous Intrusion. Am. Jour. So., April, 1903. 



2. Natural Historj' of Igneous Rocks, p. 82. et seq. 



:i. Eruptivegestcine des Krittianiagebietts, II, pp. 116-153. 

 4. Op. cit. p. 77. 



