364 PETROLOGICAL ABSTRACTS AND REVIEWS 
by the absence of such transitional types as would speak for the production 
of one from the other through differentiation. 
The second problem of petrogenesis discussed is “the causes of the 
diversity of igneous rocks.”” The processes invoked are differentiation 
and assimilation. The author does not seem to realize that we are just 
beginning to acquire the data needed for an understanding of the actual 
chemical and physical processes involved in what he discusses so con- 
fidently. Eutectic magmas are spoken of as though well known. The 
process by which a non-eutectic magma may separate into its eutectic 
and various parts representing the dissolved substances is not considered. 
No creative suggestions like those characterizing Vogt’s writings are 
found in this paper. 
That assimilation of foreign matter has disturbed the natural course 
of differentiation to a great extent is Loewinson-Lessing’s main conten- 
tion. He cites a list of advocates of assimilation, but the main argu- 
ment for it is contained in the question raised, ‘‘Is not assimilation a 
phenomenon that must be expected a priori in intrusive bodies, for it is 
difficult to imagine a magmatic basin heating the rocky masses in contact 
with it for a long period without partly dissolving them?” It is indeed 
necessary to recognize that where conditions of fusion exist that form of 
solution will take place, and further that where such conditions do not 
exist the fusion will not occur. Conditions we do not yet understand set 
limits for this process, not the range of a petrologist’s imagination. 
That many magmas represented in rocks open to investigation came 
from depths where the conditions of fusion and assimilation existed is 
seemingly incontestable. For magmas or rocks of such origin the course 
of differentiation producing them may have been decidedly interrupted 
and one can readily conceive of various plausible conditions greatly 
complicating the genetic problem. In this very situation lies the basis 
of the view, held by the present writer, that petrogenetic classification 
for all rocks must remain impracticable. 
But the assumption that assimilation has taken niece generally in 
large intrusive bodies at contacts now visible is not plain to many 
petrologists. Loewinson-Lessing’s simple statement that “In an 
intrusive body of laccolithic type assimilation takes place in the manner 
of stoping as elucidated by Daly” illustrates the ease with which he 
accepts generalizations favorable to his theories. The origin of nepheline 
syenite from granitic magmas through the reactions connected with 
assimilation of limestone, as hypothecated by Daly, is also accepted by 
Loewinson-Lessing as assisting him out of a recognized dilemma. 
