154 EXOGGERA GRANITE AND ALLIED INTRtJSIVES. 



an occasional crystal of pyrites. Small patche ; of 

 tourmaline are also found in the schist over fifty yards 

 from the contact. 



A noticeable textural change in addition to that from 

 schist to hornfels already described, and one that is well 

 developed round the Green Hill Area, is that from the 

 typically schistose to the typically gneissose. This is well 

 shown in the creek bed in the south-east corner of portion 

 310, parish Indooroopilly. Here, when about one hundred 

 yards from the contact with the Green Hill granite, the 

 schist takes on a gneissose character. The ferro-magnesian 

 minerals are gathered together in long, black, well-defined 

 bands var\-ing from less than one millimetre to over two 

 centimetres in width. The.se bands appear under the 

 microscope as masses of small fresh crystals of biotite, 

 the rest of the rock being occupied by lenticular .streaks 

 of white quartz, which, when examined by the same means^ 

 is seen to have undergone recry.stallisation. 



Chemical changes resulting from absorption of the 

 invaded schists by the granites, or allied phenomena, seem 

 to be almost entirely wanting. With but one exception, 

 the contact between the schist and the igneous rock is 

 very sharp and clearly defined. There seems to be no 

 evidence to show that assimilation of the invaded for- 

 mation by the intruding magma has taken place. The 

 one possible exception is seen in the creek bed in por- 

 tion 229. parish of Indooroopilly, where between the 

 granite — which is considerably less acid than normally 

 at this point — and the hornfels — by which the schist is 

 here represented — there is a distinct glassy band of a 

 yellow colour and measuring half an inch in thickness. 

 On examination with the microscojje this band proves to 

 be made up of irregular crystals of quartz one-half to three 

 millimetres in length arranged at right angles to the line 

 of contact, with — filling the spaces between them — aggregates 

 of a sericitic mica. The granite in the immediate neigh- 

 bourhood of this band is considerably darker than the 

 normal pink type. This is due to the facts that the aplitic 

 groundmass has almost disappeared and the proportion of 



