262 W. J. MILLER MAGMATIC DIFFERENTIATION AND ASSIMILATION 



affected by the intruded magma. ... It appears from a study of intruded 

 igneous rocks that they were not sufficiently heated to melt or dissolve invaded 

 rocks to any appreciable, or at most to any considerable, extent." 



Some of the facts very favorable to magmatic assimilation in the Adi- 

 rondack region are the following: 



(1) The tremendous masses of the syenite-granite intrusive body; 



(2) The vast number of masses of Grenville rocks which were either 

 actually caught up as inclusions or almost completely enveloped by the 

 magma, these masses ranging in size from an inch or less in width and a 

 foot or two long, to others some miles wide and many miles long ; 



(3) The very deep-seated conditions under which the Grenville rocks 

 were intruded by the magma: and 



(4) The at least frequent highly fluid character of the magma, as 

 shown by the very intimate and minute penetration of certain of the in- 

 vaded rocks by the magma. 



That the stoping and engulfment of such small to large Grenville 

 masses by the tremendous body of intruding magma were very common 

 processes throughout the Adirondack region is abundantly established by 

 recent detailed geological surveys. Also that many of these masses must 

 have sunk deep into the magma is proved by the persistent occurrence of 

 such inclusions throughout the region, irrespective of existing differences 

 of thousands of feet of altitude and the variable amount of the profound 

 denudation to which this region has been subjected. 



In spite of these very favorable conditions, there is no positive evidence 

 whatever that great bodies of the Adirondack syenitic or granitic magma 

 have been appreciably changed in composition due to the incorporation 

 or assimilation of Grenville gneisses or other rocks. As the above de- 

 scribed examples show, however, magmatic assimilation has been of com- 

 mon occurrence, but always of pretty local extent. Of the large number 

 of definitely known cases very few involve areas as large as a few square 

 miles, while by far most of them involve masses or belts only a few feet 

 or rods in Avidth and less than a mile in length. 



The differential character of the assimilation process is also note- 

 worthy. Thus at one place there will be unquestioned evidence of assim- 

 ilation, while within a stone's throw inclusions of similar rock may have 

 been enveloped by the same magma with no apparent sign of fusion. 

 These latter inclusions were doubtless enveloped by the magma when its 

 temperature was too low to bring about fusion. 



In the Adirondacks, assimilation products appear to have been pro- 

 duced in at least three ways: (1) By the so-called ^^it-par-lit" and 



