THE ANORTHOSITE BODY IN THE ADIRONDACKS 507 



In addition the anorthosite is cut by dikes of syenite in several 

 localities, some of them four or five miles in from the anorthosite 

 border. The field evidence seems clear that the anorthosite had 

 solidified, with a chilled border, and had then been attacked from 

 the side by a mass of molten syenite, which in places cut deeply 

 into it. Along the contact a basic border phase of the syenite was 

 produced, which is not a chilled border because found only along that 

 part of the syenite boundary which is in contact with the anorthosite, 

 and hence must be due to the assimilative incorporation of anortho- 

 sitic material. The product is an intermediate rock, but inter- 

 mediate between syenite and gabbro rather than between syenite 

 and anorthosite. It differs from the normal syenite chiefly in its 

 large content of ferromagnesian minerals rather than by pro- 

 nounced difference in the character of the feldspar. It somewhat 

 resembles the gabbro, but ability to distinguish the two is quickly 

 attained in the field. 



Dr. Bowen's suggested interpretation of these relations is that 

 disturbance occurred during solidification of the sheet-like mass, 

 after the anorthosite had become practically solid, but while the 

 overlying syenite was still fluid, faulting the one against the other 

 and thus permitting the fluid to laterally attack the solid, giving 

 rise to the intrusive features found in the field. This is a possible 

 cause of such relationships, but it seems to me that the presence 

 of the chilled gabbro border of the anorthosite is fatal to its applica- 

 tion in this particular case. That border seems to me to indicate 

 that this is the original size of the anorthosite mass; that it cannot 

 therefore extend westward underneath the bordering syenite; that 

 it cannot possibly underlie the great number of other syenite bodies 

 which range away for distances exceeding 50 miles to the west and 

 south. 



The presence of anorthosite outliers in the region somewhat 

 complicates the problem, and might be thought to lend support 

 to Bowen's conception of the structure. The largest of these known 

 to me is that at Rand Hill, Clinton County, which I described years 

 ago. This lies 20 miles distant from the nearest part of the 

 main body, at Keeseville, and seems to me to represent a distinct 

 intrusion, though in all probability an offshoot from the same 



