FJii/singraphy of Bulla Area. 337 



A large number' of aii^mlar and irregular rock blocks are found 

 embedded in the granodiorite along Deep Creek, 8.W. of Bulla. 

 That they were originally blocks of sedimeiit that were displaced 

 by the st'oping, and then sank into tlie molten magma, might l3e in- 

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

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

 •outlines to Ije rounded. 



Economics. — Ihe best granodiorite for building purposes lies 

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

 the 500 ft. level. 1 lie expense of liauling blocks of granodiorite 

 across these deep trenches and thence to Melbourne practically 

 prohibits the use of this rock as a building stone. 



The granodiorite. in striking contrast to the ]>asalt of this area, 

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

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



Kaalinized Grnnodlorite. 



Locaf ion .—^The granodiorite at several places round Bulla and 

 Br-oadmeadows has l^een kaolinized. In the area under discussion 

 tliere are four extensive masses of kaolinized granodiorite, and 

 several smaller outcrops. Two of the large outcrops are being 

 MX)rked by Cornwells for their Brunswick pottery. The others have 

 not yet been opened up. 



De.^cr ipf ion. —A.t 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 Avhite 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 quari-y (Qo-) 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| inches thick has lately Ijeen cut out of tlie kaolin in Qi. 



The quartz granules of the original granodiorite persist, appar- 

 ently unaltered, throughout tlie kaolinized mass. 



Microscopic Exami nation. — Angular quartz is surrounde>d by 

 crystallised kaolin, which appears chiefly as twinned lamellae, 

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

 •amount of sericite is still present, Avith earthy calcite and zircons. 



