434 W. J. MILLER ADIRONDACK AXORTHOSITE 



According to Bowen's hypothesis, these isolated masses of syenite might 

 be 'regarded as remnants of a former cover of syenite over the anorthosite. 

 But, although positive proof for their intrusive character is, so far as I 

 know, lacking, it is much more reasonable to interpret them as intrusive 

 into the anorthosite. The field relations are in most cases clear enough 

 to make it certain that these syenite masses are rather sharply separated 

 from the anorthosite, which would not be the case if they were differen- 

 tiates of the same intruded magma. 



RELATION OF THE SYENITE-GRANITE SERIES TO CHILLED BORDER OF THE 



ANORTHOSITE 



For the Long Lake quadrangle Cushing^^ says : "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." With this statement I am in agreement; 

 but I would go further in saying that both granite and syenite of the 

 syenite-granite series have, in certain other districts like the Lake Placid 

 and Schroon Lake quadrangles, not only cut deeply into, but also they 

 have either largely cut out or more or less assimilated, the border facies 

 of the anorthosite. Thus in the Schroon Lake quadrangle only mere 

 remnants of the original wide border facies are left. 



Gushing further maintains that the chilled border is fatal to Bowen's 

 conception that molten overlying syenite may have been faulted down 

 against solid anorthosite, so that it could have laterally attacked the 

 anorthosite, thus accounting for the intrusive features, including the 

 syenite dikes. From the detailed field evidence above presented, under 

 the caption "The gabbroid chilled border facies and its significance,'^ it 

 is very certain that the chilled border grades directly into the typical 

 anorthosite, and that there is not the slightest reason to think that the 

 syenite or granite ever developed between the chilled border and the 

 typical anorthosite. Two seeming exceptions should be mentioned. One 

 is the broad belt of syenite which separates the Whiteface type from the 

 Marcy type of anorthosite north of Lake Placid, but, as shown below, it 

 certainly was intruded into this position. The other is a nearly circular 

 area over a mile in diameter a few miles north of Elizabethtown, but the 

 very shape of this mass strongly points to an intrusive origin rather than 

 to its origin as a differentiate in situ. Even if we assume, what has not 

 been found in the field, that some such syenite or granite exists as a rock 

 intermediate between the chilled border and the typical anorthosite, it is 



38 H. p. Gushing : Jour. Geol., vol. 25, 1917, p. 507. 



