BY W. H. BRYAN, B.SC. 157 



the Overhead Stopmg Hypothesis, since this postulates a 

 fracturing of the invaded rock followed by the injection of 

 long sinuous apophyses of low viscosity into the fractures, 

 as the essential features of the growth of the intrusion. 



The ground-plan is that of a typical laccolite. as are 

 the proportionate lengths of the major and minor axes, 

 that is 3 : 2.^ While it is impossible from the limited 

 vertical section exposed to observation to classify this 

 igneous body strictly according to its shape — as Daly would 

 have it — these facts taken into conjunction with the probable 

 method of intrusion seem to point to the shape of this occur- 

 rence as that of a true laccolite. Similar laccolites of 

 Plutonic rock have been described from a number of places,^ 

 while Brogger explains much larger areas of granite as lac- 

 colites. 



In the considerably smaller Green Hill Area, the phenoni 

 ena seem to resemble more closely those which one would 

 expect as the result of " overhead stoping." The dips 

 of the schists seem to be almost independent of the granite 

 mass. Inclusions are fairly common and answer very well 

 to the description of those found in Daly's type localities, 2 

 but here again the Zone of Apophyses is wanting. Assimi- 

 lation is present in this area but on such an exceedingly 

 small scale as to show its practical insignificance to the 

 problem in hand. Yet even in this area mechanical energy 

 has played an important part as witness the schist breccia 

 with its finely-ground base which is so characteristic a 

 feature of this intrusion. 



Age of the Granites — Unfortunately, work done up to 

 the present can throw very little direct light on the exact age 

 of the Enoggera granites. They undoubtedly intrude 

 the Brisbane schists, as the various contact phenomena 

 plainly show. Further, the included fragments in the 

 granite are inclusions of schists, not of unaltered sedi- 

 ments, so that the movements producing the schistosity 

 occurred before the intrusion of the granite. Again, the 

 axis of intrusion cuts across the axis of folding, the two 



1. G. K. Gilbert, Geology of the Henry Mountains. 



2. Mechanics of Igneous Intrusion, p. 272. 



3. Alfred Barker Op. cit., pp. 65, 67 and 82. 



