254 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



product of digestion. It is a net result of rearrangements brought about in 

 the compound magma of assimilation. In the magma, intrusion currents and 

 the currents set up by the sinking or rising of xenoliths must take a part in 

 destroying any simple relation between the chemical constitutions of the intru- 

 sive and invaded formation. Still more effective may be the laws of differen- 

 tiation in a magma made heterogeneous by the absorption of foreign material 

 which is itself generally heterogeneous. The formation of eutectic mixtures, 

 the development of density stratification, and other causes for the chemical 

 and physical resorting of materials in the new magma ought certainly to be 

 regarded as of powerful effect in the same sense. 



A second fundamental principle has, as a rule, been disregarded in the 

 discussions on magmatic assimilation. If differentiation of the compound 

 magma has taken place so as to produce within the magma chamber layers 

 of magma of different density, the lightest at the top, the heaviest at the 

 bottom, the actual chemical composition of the resulting rock at any contact 

 will depend 'directly on the magmatic stratum rather than on the composition 

 of the adjacent country-rocks. 



In the foregoing discussion the secondary origin of some granites has 

 been deduced from the study of intrusive sills or sheets; but it is evidently 

 by no means necessary that the igneous rock body should have the sill form. 

 The wider and more important question is immediately at hand — does the 

 assimilation-differentiation theory apply to truly abyssal contacts? Do the 

 granites of stocks and batholiths sometimes originate in a manner similar or 

 analogous to that just outlined for tire sills? 



General reasons affording affirmative answers to these questions are noted 

 in chapter XXVI. 



Gabbro and granophyre are often characteristically associated at various 

 localities in the British Islands as in other parts of the world.f The field rela- 

 tions are there not so simple- as in the case of the Moyie sills, for example, 

 but otherwise the recurrence of many common features among all these rock- 

 associations suggests the possibility of extending the assimilation-differentia- 

 tion theory to all the granophyres. Barker's excellent memoir on the gabbro 

 and granophyre of the Carroekfell District, England, shows remarkable 

 parallels between his Maccolite' rocks and those of Minnesota and Ontario:}: 



At Carrock Fell there is again a commonly occurring transition from the 

 granophyre to true granite, and again the granophyre is a peripheral phase. 

 Still larger bodies of gabbro, digesting acid sediments yet more energetically 

 than in the intrusive sheets, and at still greater depth, would yield a thoroughly 

 granular acid rock as the product of that absorption with the consequent- 

 differentiation. 



The difficulty of discussing these questions is largely owing to the absence 

 of accessible lower contacts in the average granite body. All the more valuable 



t See A. Geikie, Ancient Volcanoes of Great Britain, 1897. 



j Quart. Journal Geol. Soc, Vol. 50, 1894, p. 311 and Vol. 51, 1895, p. 125. 



