GEOLOGY OF ERUPTIVE AND ASSOCIATED ROCKS, POKOLBIN. 395 



followed on the rhyolite apparently without any consider- 

 able interval. Succeeding the trachyte was a period of 

 quiescence in the vulcanicity and of gradual subsidence, 

 during which the Carboniferous conglomerates, shales and 

 sandstones were laid down, before volcanic action recom- 

 menced. The longest period of volcanic inactivity was 

 probably that between the dacite and the Lower Marine 

 basalt. During this interval the Permo-Oarboniferous 

 sedimentation began and advanced to a considerable degree. 



Petrology. 



The rocks found in the district include, as has been 

 already shown, a wide range of types. They may be divided 

 up as follows : — 



A. Plutonic, 



B. Hypabyssal, and 

 0. Volcanic (i) acid 



(ii) intermediate, and 

 (iii) basic. 



The only plutonic representative is the grano-diorite mass 

 exposed along the eastern face of Mount Bright. Hypabyssal 

 rocks are represented by the dolerite at Matthews' Gap 

 which occurs as a (?) volcanic neck, and the small neck of 

 ophitic dolerite in the Mount Bright area. The acid vol- 

 canics include the rhyolites and the dacite, both occurring 

 as flows, and probably the leucocratic trachytes can be 

 classed with these. The intermediate volcanics include 

 the melanocratic trachyte (which in hand-specimens 

 resembles very closely a light coloured-basalt), trachy- 

 andesite, andesite and pitchstone. Among the basic rocks 

 there are olivine basalt, and basalt. 



One of the most noticeable features of the volcanic series 

 is the extremely limited development of ferro-magnesian 

 constituents. This is very marked in the trachytes and 



