THE ANORTHOSITE BODY IN THE ADIRONDACKS 505 



demonstrate that the full girth of the anorthosite intrusion is 

 represented by the dimensions shown on the present maps and that 

 the outlying syenite bodies represent distinct and slightly later 

 intrusions. 



Dr. Bowen 1 discusses the "Intimate Relation of Syenite and 

 Anorthosite" and makes the following statement: 



This aspect of the anorthosite, i.e., its intimate connection with the syenite, 

 is emphasized in the area as a whole, where, in spite of fairly good exposures, 

 only one other locality showing the intrusive relation of syenite to anorthosite 

 has been found, but where, on the other hand, types intermediate between the 

 two are rather commonly found. 



It is chiefly these two points of his paper which I wish to discuss, 

 since my field experience is quite antagonistic to them. I have 

 myself published several localities where dikes of syenite cut the 

 anorthosite, and on the next page of his paper Bowen quotes W. J. 

 Miller as authority for the statement that dikes of syenite cutting 

 anorthosite occur in the Placid region. My thesis is that the 

 general differentiation in situ shown by the anorthosite is into 

 anorthosite-gabbro and gabbro, and not into syenite; that such 

 intermediate rocks as do occur are chiefly intermediate between 

 syenite and gabbro, instead of between syenite and anorthosite; 

 and that the demonstrable source of these latter intermediate rocks 

 in many cases, if not in most or all of them, is assimilative attack 

 of a later intrusion upon an earlier, and is not differentiation 

 in situ. 



The boundary of the anorthosite is in part along faults. Where 

 unfaulted the anorthosite is always found to grade into anorthosite- 

 gabbro, and this into gabbro as the boundary is approached. This 

 change is depicted upon the Long Lake and the Elizabethtown 

 quadrangle maps and occurs also in all other parts of the region in 

 which I have any acquaintance with the boundary. Daly has 

 interpreted this as a chilled border of the anorthosite, and in my 

 judgment this is not only the most reasonable, but in fact the only 

 satisfactory, explanation that can be made of it. 2 If this be true, 



1 Op. cit., p. 211. 



2 Igneous Rocks and Their Origin, p. 240. 



