PJtysiography of Bulla Area. 337 



A large numben of angular and irregular rock blocks are found 

 embedded in the granodiorite along Deep Creek, S.W. of Bulla. 

 That they were originally blocks of sediment that were displaced 

 by the stoping, and then sank into the molten magma, might be in- 

 ferred by the angularity of the blocks, and by their close similarity 

 to hornfels. If they were basic segregations, you would expect the 

 outlines to be rounded. 



Economics. — the best granodiorite for building purposes lies 

 between the deep trenches of Jackson's Creek and Deep Creek, at 

 the 500 ft. level. The expense of hauling blocks of granodiorite 

 across these deejD trenches and thence to Melbourne practically 

 prohibits the use of this rogk as a building stone. 



The granodiorite, in striking contrast to the basalt of this area, 

 is always tree-covered, and with the exception of the trees in the 

 deep creek trenches, is the only local source of timber. 



Kaolinized Granodiorite. 



Location. — The granodiorite at several places round Bulla and 

 Broadmeadows has been kaolinized. In the area under discussion 

 there are four extensive masses of kaolinized granodiorite, and 

 several smaller outcrops. Two of the large outcrops are being 

 ATOrked by Cornwells for their Brunsw^ick pott-ery. Ihe others have 

 not yet been opened up. 



Description.— kt the quarry Q" (Plate XXXII.) one can trace the 

 change from hard granodiorite, through the partly decomposed to 

 the thoroughly decomposed and whitened rock. Decomposed basic 

 segregations can also be seen in the face. Much of the mass is left 

 the purest white by the leaching out of the iron oxide derived from 

 the magnetite, pyrite, biotite and chlorite. In other parts the 

 decomposed rock is deeply stained and cemented by the concentra- 

 tion of iron! oxide. 



In the smaller quarry (Q2) near the Bulla school, there is a well- 

 marked vein one inch thick, of bluish tourmaline and granular 

 quartz, in a joint plane of the kaolinized granodiorite. Another 

 vein 1 J inches tliick has lately boen cut out of the kaolin in Qi. 



The quartz granules of the original granodiorite persist, appar- 

 ently unaltered, throughout the kaolinized mass. 



Microscopic Examination. — Angular quartz is surrounded by 

 <'rystallised kaolin, which appears chiefly as twinned lamellae, 

 though often in the form of scales and aggregates. A considerable 

 :amount of sericite is still present, with earthy calcite and zircons. 



