244 W. J. MILLER MAGMATIC DIFFERENTIATION AND ASSIMILATION 



tion. Probably few regions present any better opportunities for the study 

 of such phenomena. It is the purpose of this paper to bring together 

 published and unpublished observations bearing on these problems and 

 to present certain conclusions which have an important bearing not only 

 on Adirondack geology, but also on the broader problems of petrology. 



Particular acknowledgments are due Professors Gushing, Kemp, and 

 Smyth, all of whose publications pertaining to the geology of the region 

 are so familiar to the writer. During the past eight years the writer's 

 work has been largely confined to the southern half of the Adirondack 

 region and has resulted in the detailed mapping of about 1,200 square 

 miles of pre-Cambrian rock. Problems of differentiation and assimilation 

 have always been prime considerations. 



It is quite generally agreed that the science of petrology is still in its 

 formulative stage. Certainly this stage will never be passed unless gen- 

 eralizations are based on a broad array of detailed observations and facts. 

 Various papers have recently appeared in which speculations and hy- 

 potheses occupy prominent places. Such papers are, of course, impor- 

 tant, but the basis of fact is often meager. Hence, in the present paper, 

 which deals with an unusually favorable region, much space is given to 

 detailed descriptions illustrating various phases of differentiation and 

 assimilation. 



This paper deals directly only with the great bodies of syenite and 

 granite and their influence on the Grenville rocks into which they have 

 been intruded in the Adirondack region. The large body of anorthosite, 

 which lies mostly in Essex County, is only indirectly referred to, while 

 the later and minor intmsions of gabbro and diabase are not discussed at 

 all. Magmatic assimilation as a prime cause of the variations of the 

 later gabbro has been discussed at some length in a recent paper by the 

 writer.^ 



Facies of the great Syenite-Granite intrusive Body 



general observations 



With certain possible rather local exceptions, the great masses of syenite 

 and granite, which are the most abundant rocks in the Adirondack region, 

 are regarded as belonging to a single, vast intrusive body. Evidences 

 for this view are presented below. What may be regarded as a normal 

 qnartz syenite shows, on the one hand, facies- which are basic and really 

 of dioritic or even gabbroic composition, while, on the other hand, there 



2 W. J. Miller : Variations of certain Adirondaclj basic intrusives. Jour. Geo!., vol. 21, 

 1913, pp. 160-180. 



