SOME GEOLOGICAL FEATURES OF NORTHERN AUSTRALIA. 117 



Bitumen was obtained by Drill-Foreman Berry in 1911 in 

 Tjoring on a silver-lead-zinc lode in Cambrian limestone country 

 on Cattle Creek, about 10 miles from Me Arthur Station. Ball 

 has recorded anthracite from the Silver King lode in the 

 Burketown Mineral District (Q.G.S. Pub. 232, p. 30). In the 

 writer's opinion these occurrences probably have no corm^ection 

 with true coal seams, but are purely of chemical origin. 



Magnetite- Hematite-Quartz Lodes or Beds. — McLaren in his 

 -work on " Gold " says : " By far the most characteristic rock 

 of the Archaean group, and one always associated with the 

 sedimentary members of the series, is a well-banded, generally 

 much-contorted, hematite-magnetite quartz rock, of obscure 

 origin. It has been thought to arise from silicification along 

 shearing planes, but it may most reasonably be regarded as due 

 to the metamorphism of ferruginous silicate and carbonate 

 bands in depth, with resultant conversion into ferric oxide and 

 silica." 



Although the writer has not diagnosed this rock type in 

 the Territory, the allied " hematite quartz " rock occurs 

 frequently in the Rum Jungle, Margaret, Mount Ellison, and 

 other districts, and, as far as the writer's experience goes, it is 

 always a shear formation and not of purely sedimentary origin. 

 In the Cairns district in a grejnvacke schist series of much later 

 (Devonian ?) age we get hematite quartz and hematite-magnetite 

 quartz rocks which are certainly shear formations, the shearing 

 having occurred on bedding planes. The original sedimentary 

 rock substance has been supplemented by silica and mineralisers 

 from below. 



The ferriferous shears are always most freely developed in 

 the Territory in regions where amphibolitic rocks abound — a 

 feature which certainly points to the derivation of much of the 

 iron through transport by mineralising solutions from the ferri- 

 ferous amphibolite to the shear zones. The shear zones are 

 channels in which metasomatism is affected by the agenc}' of 

 mineralisers carrying iron from basic and ultra-basic schists 

 and intrusives, which they are simultaneously metasomatising. 



Thus, one may reasonably deduce that the frequency of 

 hematite-magnetite c^uartz zones in the Archaean is a sequence 

 of the abundance of basic and ultra-basic intrusives ino thes 

 formations. 



R.S. — I. 



