BY W. N. BENSON. 589 



gin, and certainly suggests that the limits of the granite-intrusion 

 may have been determined by the tectonic structure of the invaded 

 rocks. It seems quite clear, however, that the granite, if laccolitic, 

 did not extend far over the Seven-Mile Creek anticline. The rocks 

 that form this structure are very little altered, while the rocks, 

 which, in Erdmannsdorfer's view(24), are to be regarded as the 

 Devonian and Carboniferous rocks altered by the overlying lacco- 

 lite of the Brocken granite, are the highly altered series, that form 

 the Eckergneiss.* 



The southern margin of the granite is quite transgressive. It 

 seems to be no more than a coincidence that part of it is approxi- 

 mately parallel to the axis of the Cockburn River syncline, for 

 elsewhere it cuts right across the axis, and also across the several 

 repetitions of the Lower Middle Devonian Series, the Serpentine 

 Belt, and invades the Eastern Series. Hence we may conclude that 

 the folding of the Devonian rocks, both along the main structural 

 N.N.W.-S.S.E. axis, and the subsidiary N.E.-S.W. axis was accom- 

 plished before the intrusion of the granite occurred. The absence 

 of any noteworthy gneissic structure along the margin of the 

 granite is also evidence towards the same conclusion. 



Tertiary Basalt. 



A small plug of basalt occurs on the western edge of Portion 

 83, Parish of Woolomol. It is roughly oval in shape, and about 

 sixty yards long. It does not give any noticeable relief. The 

 rock is granular or aphanitic, with small crystals of olivine, and 

 is massive or slightly vesicular. The microscof)e reveals the 

 presence of cognate xenoliths of olivine, pyroxene, arid picotite, 

 and very interesting vesicles filled with natrolite and opal, which 

 seem to be of primary origin. In general character, the rock is 

 quite analogous to the majority of the Tertiary basalts in other 

 parts of the State. 



Terrace-Gravels. 



On either side of the Cockburn and Peel Rivers, may be seen 

 terraces of gravels lying thirty or forty feet above the present 



* Goldschmidt, however, doubts the possibility of such gneisses being 

 produced under so small a pressure as Erdmatnisdorfer assumes(26)- 



