REPORT OF THE CHIEF ASTRONOMER 255 



SESSIONAL PAPER No. 25a 



must be the information derived from intrusive sills. The comparative rarity 

 of such rock-relations as are described in this chapter does not at all indicate 

 the exceptional nature of the petrogenic events signalized in the Moyie, Pigeon 

 Point, or Sudbury intrusives. It is manifest that extensive assimilation and 

 differentiation can only take place in sills when the sills are thick, well buried, 

 and originally of high temperature. All these conditions apply to each case 

 cited in this chapter. The .phenomena described are relatively rare largely 

 because thick basic sills cutting acid sediments are comparatively rare. 



On the other hand, there are good reasons for believing that a subcrustal 

 gabbroid magma, actually or potentially fluid, is general all around the earth; 

 and secondly, that the overlying solid rocks are, on the average, gneisses and 

 other crystalline schists, and sediments more acid than gabbro. Through local, 

 though widespread and profound, assimilation of those acid terranes by the 

 gabbro, accompanied and followed by differentiation, the batholithic granites 

 may in large part have been derived. True batholiths of gabbro are rare, 

 perhaps because batholithic intrusion is always dependent on assimilation. 



The argument necessarily extends still farther. It is not logical to restrict 

 the assimilation-differentiation theory to the granites. For example, the prepar- 

 ation of the magmas from which the alkaline rocks have crystallized, may have 

 been similarly affected by the local assimilation of special rock-formations. 

 See chapter XXVIII. 



The officers of the Minnesota Geological Survey have shown that the same 

 magma represented in the soda granite and granophyre of Pigeon Point forms 

 both dikes and amygdaloidal surface flows* The assimilation-differentiation 

 theory is evidently as applicable to lavas as to intrusive bodies. But demon- 

 stration of the truth or error of the theory will doubtless 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 petro- 

 graphical province may be due as much to assimilation as to differentiation. 



* N. H. Winchell, Final Rep. Minn. Geol. Surv., Vol. 4, 1899, pp. 519-22. The Duluth 

 gabbro and tbe broad fringe of red rock (partly extrusive) on the southeast, together 

 seem to form a gigantic replica of the Pigeon Point intriisive! 



