46 PROCEEDINGS OF THE PRINCETON MEETING 



was the purpose of this paper to briefly summarize the more important obser- 

 vations made by the writer during the past eight years bearing on these 

 problems. . 



Viewed In the broadest way, the great masses of plutonic rocks, excluding 

 the anorthosite, show all sorts of gradations back and forth, from gabbro and 

 diorite to granite and granite porphyry, with a quartz syenite the prevailing 

 type. In this connection the possible presence of a great intrusive (so-called 

 "Laurentian granite") older than the syenite in the Adirondack region will 

 be discussed. 



Besides the above-mentioned plutonic rocks, ■ there are an almost endless 

 number and variety of rocks which are neither simple intrusives nor true 

 Grenville,, but which are distinctly intermediate in character. 



Evidence was presented to show that the broader variations among the 

 unquestioned intrusives is due to differentiation practically unaccompanied by 

 assimilation ; that more locally, though often to a considerable extent, pretty 

 complete assimilation of Grenville gneisses by the syenitic and granitic magmas 

 has produced many rocks of intermediate character, and that in very many 

 cases the magmas enveloped Grenville gneiss fragments and either fused only 

 their edges or left them completely undissolved. 



Read in abstract from manuscript. 



Discussion 



Dr. Frank D. Adams : The phenomena described by Messrs. Fenner and 

 Miller are identical with those displayed in the Laurentian-Grenville areas as 

 the margin of the Canadian shield in Canada. In the Haliburton and Ban- 

 croft areas of eastern Ontario it is possible to definitely prove that the dark 

 inclusions in the gneissic granite of the batholith are in very many cases at 

 least fragments of the wall rock which have been torn off the invading rock. 

 There is strong presumption that the similar fragments found in the inner 

 portions of the batholiths are derived from the roof of the batholith. Only 

 in a very few places in that great region of 4,500 square miles could proof be 

 secured that an actual solution of its wall rock in the granite magma had 

 taken place, and these areas were of very limited extent. 



Mr. Sidney Paige : A pegmatitic batholith in the Black Hills, South Dakota, 

 throVs light on the mechanics of granite intrusions in general. The tentative 

 conclusion has been reached that the intrusion has entered by virtue of its 

 ability to deform a series of previously metamorphosed sediments, to form 

 in these sediments close recumbent folds, and to inject these sediments along 

 the new schistosity developed. Lit-par-lit injection combined with actual mass 

 deformation of the schists are the mechanical elements involved. Stoping on 

 any but a small scale is absent. Assimilation is confined to the immediate 

 borders of the granite contact and is believed to be lit-par-lit injection on a 

 minute scale plus solution of the schist. The introduction of the granite in 

 the layers of the schists is a move in the direction of bringing the schists and 

 granite to equal specific gravities, and this mitigates against the sinking of 

 stoped blocks. 



Further remarks were made by Professor Kemp. 



