216 JR. A. Daly — Secondary Origin of Certain Granites. 



more acid than gabbro. Through local, though widespread 

 and profound, assimilation of those acid terranes by the gab- 

 bro, accompanied and followed by differentiation, the batholithic 

 granites may in large part have been derived.* True batho- 

 liths of gabbro are rare, perhaps because batholithic intru- 

 sion is always dependent on assimilation. 



The argument necessarily extends still farther. It is not 

 logical to restrict the assimilation-differentiation theory to the 

 granites. The preparation of the magmas from which syenites 

 and diorites, for example, have crystallized, may have been 

 similarly affected by the local assimilation of special rock- 

 formations. The development of some of the anorthosites of 

 the Canadian and Adirondack Archean was possibly condi- 

 tioned on the digestion of part of the associated crystalline 

 limestones by plutonic magma. 



The officers of the Minnesota Geological Survey have shown 

 that the same magma represented in the soda granite and grano- 

 phyre of Pigeon Point forms both dikes and amygdaloidal 

 surface flows, f The assimilation-differentiation theory is evi- 

 dently as applicable to lavas as to intrusive bodies. But 

 demonstration of the truth or error of the theory will doubt- 

 less be found in the study of intrusive igneous bodies rather 

 than in the study of volcanoes either ancient or modern. 



Finally, the fact of " consanguinity" among the igneous rocks 

 of a petrographical province may be due as much to assimilation 

 as to differentiation. 



* Cf . K. A. Daly, op. cit. 



fN. H. Winchell, Final Eep. Minn. Geol. Surv., vol. 4, 1899, pp. 519-22. 



