Geology of B roadmeadows. 161 



at heavy expense to Northcote and to Ringwood, and used suc- 

 cessfiillT for tire bricks and for lining furnaces. This outcrop, 

 being nearer to Melbourne, may therefore be of value. 



Associated Dykes. 



In Section YIII., Will Will Rook, there occurs a line of boul- 

 ders of quartz porphyry in the silurian country. The line is 

 east and west, but is determined by a fence of a cultivated pad- 

 dock. But it is evident that a quartz porphyry dyke occurs in 

 the vicinity, which has shed these boulders in weathering. The 

 rock consists of phenocrysts of quartz and felspar set in a very 

 fine-grained ground-mass. The felspar has a rectangular outline, 

 but is too decomposed to be retained in thin section. It is 

 probable that this quartz porphyry dyke occurring so close to 

 the contact of the silurian and granite is to be associated with 

 the intrusion of the granite. 



Chemical Characters. 



A chemical analysis of this rock was carried out in April, 

 1909. at the Geological Laboratory of the Melbourne University, 

 by Mr. H. S. Richards, M.Sc. The analysis has not been pub- 

 lished hitherto, and it is Mr. Richards' generosity that makes it 

 available noAv for discussion along with the petrographical 

 characters. 



The silica is lower^i and the lime and magnesia higher than 

 in a true granite. Still the total lime and magnesia is lower 

 and the total alkalies higher than the Macedon grano-diorites. 

 The total composition suggests that it is nearer to the adamellite 

 type than the true grano-diorite. This suggestion is supported 

 by the petrographical examination. The relative amounts of 

 plagioclase and orthoclase were determined by a series of 

 traverses across a rock slice, and it was found that the propor- 

 tion of plagioclase to orthoclase was less than two to one. The 

 position of the rock in the American system is — Class I., Persa- 

 lane ; Order 4, Brittanare ; Rang 2, Toscanase ; Sub-Rang 3, 

 Toscanose. 



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