Ordovician Sandstone. 69 



magmatic stoping, assimilating the country rock as it progresses. 

 The particular inclusion at Big Hill would be looked upon as a 

 fragment of Ordovician which had not been entirely digested, 

 whilst the surrounding basic segregation would be explained as 

 granite which in the immediate neighbourhood of the Ordovician 

 had its composition altered by solution of the sedimentary rock. 

 Under normal circumstances of slow diffusion, the alteration of 

 composition of the granite would be a gradual and progressive 

 one, from a maximum at the surface of the country rock out- 

 wards to the normal granite. The acute change of composition 

 of this segregation at the margins would, however, be explained 

 by picturing the fragment as being localised to a certain definite 

 neighbourhood until a state of equilibrium was reached so far 

 as slow diffusion was concerned. For some short distance around 

 the fragment of sandstone, the granite magma would now be 

 of an approximately similar composition throughout. If now 

 further movement of the magma took place, so that the position 

 of the fragment were altered — as, for instance, if the sandstone 

 commenced to sink — then this equilibrium would be immediately 

 disturbed. A certain amount of the surrounding altered magma 

 would be carried with the inclusion and corroded by the magma 

 to some extent, until finally, the whole granitic intrusion crystal- 

 lized out. 



On this hypothesis of magmatic stoping and marginal assimi- 

 lation it would, as a necessary corollary, be presupposed that the 

 segregation immediately surrounding the fragment would be of 

 a composition intermediate between that of the granite, and in- 

 cluded country rock, i.e., an inclusion of higher SiO^ content 

 than the granite should give a surrounding segregation of more 

 acid composition than the granite, while a more basic inclusion 

 would give a segregation of less acid composition. The uphold- 

 ing of this supposition by chemical analysis- would go a long way 

 towards the acceptance of the hypothesis, whilst a negative result 

 from chemical analysis would mean the absolute rejection of the 

 hypothesis of marginal assimilation so far as this occurrence is 

 concerned. The following are the average silica contents of the 

 three rock types as shown by analyses kindly undertaken by 

 Miss Mclnerny, of the Geology School, University of Mel- 

 bourne : — 



Country rock (sandstone) .... 78.30 per cent. SiO^ 



Segregation 65.30 per cent. SiO^ 



Granite 69.79 per cent. SiO^ 



