208 R. A. Daly — Secondary Origin of Certain Granites. 



pressure have specific gravities varying between 2*60 and 2*65. 

 There are good reasons for believing that plutonic pressures 

 would not essentially affect these contrasts of density. Assum- 

 ing a certain degree of fluidity in the magma (an assumption 

 underlying the whole of this paper and believed to be demon- 

 strated by such facts as the patent ease of diffusion that once 

 reigned in each of the intrusives), it appears that blocks of the 

 sedimentary rocks must sink in the magma, whether acid or 

 basic* 



The actual shatter-breccias described by Bayley are there- 

 fore to be attributed to the last destructive effort of the magma, 

 which, at that time, through cooling, had become too viscous 

 to allow of the sinking of the xenoliths. 



A precisely similar argument applies to the Moyie and Sud- 

 bury examples (see table of specific gravities and table in this 

 Journal, vol. xv, 1903, p. 277). All these igneous bodies, 

 though not intruded by magmatic stoping, yet show that pro- 

 cess to have assisted in the production of the granites and 

 granophyres. Whether this process has there been more or 

 less efficaceous than molar or marginal assimilation, perhaps 

 cannot be determined. 



In all these cases the stoping that did occur must clearly 

 have tended to destroy a simple chemical identity between 

 igneous rock and country-rock at any given contact. 



Summary. — It will be useful to review the chief field and 

 laboratory observations so far noted as favoring the assimila- 

 tion theory when applied to the granites and granophyres 

 described in this paper. 



1. JBayley's elaborate argument is believed to be valid except 

 as it fails to take differentiation into account. No fact has been 

 noted either by the writer in connection w T ith the Moyie sill or 

 in the descriptions of the other examples which tends to weaken 

 that argument. 



2. Belief in the truth of his conclusion is greatly strengthened 

 by the repeated occurrence of essentially the same phenomena 

 in widely separated regions. 



a. At Pigeon Point, at Sudbury and on the Moyie Piver 

 there occur intrusive bodies of gabbro passing by gradual tran- 

 sitions (as shown by chemical, mineralpgical and specific gravity 

 determinations) into the border phases of granite and grano- 

 phyre. Both types of rock clearly belong to the same period 

 of intrusion. 



b. All three igneous bodies are of relatively great thickness, 

 which means that, other things being equal, they possessed 

 relatively great stores of thermal energy. 



*Cf. E. A. Daly, op. cit., 1903, p. 277, etc. 



