570 N. L. BOW EN 



with liquid rich in anorthite or magnesian siHcates, alumina is set 

 free as corundum. This condition is transient, however, and even 

 these residues from the inclusions may become a part of the general 

 mass as a result of the reactive precipitation noted above. The net 

 result is the formation of noritic material with an increase in amount 

 of the acidic differentiate normally possible. 



The addition of limestone to basaltic magma may perhaps give 

 rise to a liquid capable of precipitating melilite in some cases and 

 from such a liquid it is possible that some alkaline rocks may form 

 by further differentiation. It does not seem possible that limestone 

 inclusions can desilicate a granitic magma in such a way as to give 

 rise to a liquid capable of precipitating feldspathoids. However, if 

 limestone inclusions were present during the differentiation of the 

 more basic liquid from which the granitic liquid may have formed, 

 the presence of such inclusions might reduce the amount of free silica 

 associated with the alkaline feldspar in this liquid to such an extent 

 that the normal tendency of orthoclase to break down into leucite 

 would manifest itself. Thus rocks bearing leucite, and possibly 

 other feldspathoids, might form, but influences prevailing during 

 early stages of differentiation, other than the presence of foreign 

 matter such as limestone, may likewise lead to the formation of 

 leucite at a late stage. 



In conclusion, it may be stated, therefore, that magmas may 

 incorporate considerable quantities of foreign inclusions, both by 

 the method of reactive solution and by reactive precipitation, and 

 such action may have been important in connection with the pro- 

 duction of certain individual masses. Thus some norites may have 

 been produced as a result of the reactions discussed above, some 

 granites may have had their mass augmented by reactive solution 

 of granitic inclusions in the magma from which they differentiated, 

 some alkaline rocks may have been formed as a result of the presence 

 of limestone inclusions in the liquid from which they differentiated. 

 All of these actions are, however, an emphasizing of normal pro- 

 cesses possible in the absence of foreign matter. It is doubtful 

 whether the presence of foreign matter is ever essential to the pro- 

 duction of any particular type of differentiate. 



Washington, D.C. 

 April 25, 1922 



