70 J. A. Dunn: Ordovician Sandstone. 



The segregation is thus not of an intermediate composition, 

 the SiO^ content being lower than either of the other two; hence 

 the hypothesis of marginal assimilation as applied to the ex- 

 planation of the origin of the segregation surrounding the Big 

 Hill inclusion of country rock must be rejected. 



(b) The theory of magmatic differentiation upheld by Bowen 

 jaffords an excellent explanation of the origin of the basic 

 " aureole " to the inclusion. This '' aureole " would represent 

 a portion of the chilled border facies attached to the roof, and 

 subsequently broken off with some of the country rock, to be 

 incorporated in the parent magma. At the intrusion of the 

 molten magma, the cooling at the marginal walls would be 

 ahead of the cooling of the main mass, and here there would 

 probably crystallize out a rock of the same composition as the 

 molten magma at that instant. The main mass of the magma 

 would remain still liquid, and as cooling progressed differentia- 

 tion by sinking of crystals would continue, the liquid magma be- 

 coming more and more acid until, ultimately, the whole would" 

 crystallize out, producing as an ideal result an acid alkalic 

 magma with a less acid and more calcic border (the chilled bor- 

 der facies). But prior to the crystallization of the main liquid 

 magma, picture a rejuvenation in the mechanical activity of the 

 magma brought about perhaps by earth movement, so that mag- 

 matic stoping commenced afresh. Picture also a small roof 

 pendant of country rock projecting into the magma, and around 

 which a " chilled border facies " has crystallized. With the 

 renewal of magmatic stoping this roof pendant and its attached 

 basic granitic border will break away from the main country 

 rock, and sink into the liquid magnia to be perhaps slightly 

 corroded, but finally isolated in the granite on solidification of 

 the magma. This will also explain the curious shape of this 

 particular inclusion of Big Hill — a fragment of country rock 

 (about six inches wide and two feet long), surrounded by a 

 basic segregation except on that side which may be pictured as 

 the area of attachment to Ordovician roof. 



This, then, is quite an acceptable explanation of the origin 

 of this inclusion, and considered as such, the Big Hill inclusion 

 affords excellent evidence of the possibilities of both Daly's 

 hypothesis of magmatic stoping, and Bowen's research on the 

 differentiation of rock magmas, particularly as applied to the 

 origin of " chilled border facies." 



