CHAPTER XIV 



ERUPTIVE ROCKS, PROBABLY POST-CAMBRIAN 



AND PRE-GONDWANA 



As already stated, there is a considerable development of folded granitic rocks in the 

 Pre-Cambrian group, and it is very difficult to difterentiate between the true Pre- 

 Cambrian gneisses and the gneissic granite or sub-gneissic granite of localities such 

 as Depot Island, Mount Crummer, &c. For example, at Depot Island there are 

 large included masses of quartzite caught up in the sub-gneissic granite, together 

 with very numerous inclusions of a basic type of sphene-bearing diorite. It is 

 possible that these masses of quartzite have been torn out of the basal portion of 

 the Beacon Sandstone. If so, this sub-gneissic granite would be Post-Gondwana 

 instead of Pre-Gondwana. For the present we are considering such granites as 

 Pre-Gondwana, looking upon the quartzites as something older than the Beacon 

 Sandstone formation. 



These eruptive rocks, following Dr. Mawson's and Professor Woolnough's classi- 

 fications, we may group as follows : — 



I. Granites and associated dyke rocks. 



1. Granites. 



2. Dyke rocks. 



(i) The porphyries. 



Granite porphyry, granophyric granite porphyry, orthoclase 

 porphyry, quartz porphyry, halleflinta-like porphyries, aplitic 

 granite porphyries, 

 (ii) The aplites. 

 (iii) The pegmatites, 

 (iv) The granophyres. 

 (v) The porphyrites. 



(vi) The lamprophyres, minettes, kersantites, &c. 

 II. Syenites, including sodalite-syenite with wohlerite. 



III. The diorites. 



Quartz diorites and basic diorites rich in sphene, the latter occur- 

 ring in places as inclusions in granite. 



IV. The gabbros. 



