160 ENOGGERA GRANITE AND ALLIED INTRUSIVES. 



siliceous, until at last they resemble very closely some 

 quartzites. A good example of such an altered rock is to 

 be seen in the quarry at Corinda. 



As regards mode of occurrence these rocks are found 

 both as dykes, sills and laccolites. Examples of the first 

 type are very common, the great majority of the smaller 

 intrusions being of this nature. Sills, though not so plentiful 

 have been observed at a number of points throughout the 

 area, the large intrusion in portion 684, parish Indooroopiliy 

 probably being of this nature. 



A good example of the laccolitic type of intrusion is 

 to be seen on the northern edge of the Mount Cootha Reserve. 

 This large mass is now considerably dissected by young 

 streams, but enough remains to show that it is a laccolite. 

 The level of its base — 470 ft. above sea level — is wonder- 

 fully constant. A small patch of schist still caps the top 

 of the intrusion which is 590 ft. above sea level, giving a 

 thickness of 120 feet for the intrusion. As far as can be 

 determined the laccolite measures about 550 yards in length,, 

 the ratio of height to length being approximately^ 1 : 14. 

 Another splendid example of this type of intrusion occurs 

 at Indoocoopilly. Here on the left bank of the Brisbane 

 River, just below the Albert Bridge, is to be seen an oblique 

 section across the end of a laccolite. The schists can be 

 observed dipping away from the intrusion on both sides, 

 while the jointing of the intrusion itself forms a pronounced 

 curve with its convexity upwards since it seems to follow the 

 outline of the intrusion itself. This particular intrusion can 

 be followed for nearly a mile in a north-westerly direction, 

 but the outcrop is merely a small portion of the top which 

 has been exposed by the erosion of the overlying schist. ♦ 



At first sight, the rhyolitic intrusions seem to lack 

 uniformity of direction, but a study of the map, together 

 with numerous observations in the field, show that there are 

 two or three directions which are particularly favoured. 

 These are west-north-west, north-west, and north-north- 

 west. Those falling under the first and second directions 

 seem to run more or less in the general direction of the 

 strike of those schists, and are well illustrated by the large 

 intrusion at Indooroopiliy just discussed. The last direct- 



