VOLCANIC NECKS OF HORNSBY AND DUNDAS. 519 



mate mineralogical composition of a series of these estimated 

 by rough comparison only of their areas in section. The 

 order of consolidation is given in the last column, where P 

 a= pleouaste, R - rhombic pyroxene, M = monoclinic 

 pyroxene, F = felspar, D = decomposition product of 

 indeterminate origin : — 



Rock. F M R P D Order of crystallisation. 



a 60 35 4 10 P pTTi P 



b 50 36 12 2 P R M F 



c 27 64 6 3 P R M F 



d 18 38 42 2 P M R F 



e 12 82 1(?)2 5* P M F 



* Probably after hypersthene. 



Estimating in the same rough way the cnniposition of the 



granophyric rocks we obtain the following figures. O = 



olivine, either fresh , or i n> 1 ic;i t< •• I I <y decomposition products. 



Rock. F M M P D 



f 96 3'5 0*5 



k 10 72 6 2 10 



From these tables it will be seen that the central mem- 

 bers of both series are richest in spinel and that in the 



granophyric series olivine is present in greatest abundai 



in the rocks lying in the centre, while absent from the 

 non-granopliyric series. Using the triangular diagram 

 again (fig. 6) we may say that the non-granophyric rocks 

 lie on the line a p the granophyric rocks in the triangle a op. 

 The structure of the intergrowth is peculiar. On first 

 sight it would appear that the felspar was moulded on the 

 pyroxene containing the pleonaste, but the fact that the 

 pleonaste fibres start perpendicularly out from the junction 



