GEOLOGY OF THE SCHROON LAKE QUADRANGLE 2? 



the form of labradorite) accumulated by sinking, thus giving rise 

 to the mass of the anorthosite, leaving the overlying liquid of such 

 composition as to yield syenite or granite. In his first paper, Bowen 

 does not consider the development of a chilled border of the Adi- 

 rondack anorthosite. In his second paper, by way of reply to 

 Gushing, he modifies his idea of the stratiform arrangement of 

 the igneous complex by considering the development of a " gab- 

 broid chilled upper portion of a laccolithic mass extending far 

 beyond the limits of the present exposure." Directly under the 

 chilled border, according to Bowen, the great body of syenite- 

 granite developed; still lower down the typical anorthosite formed; 

 and at the bottom, pyroxenite and gabbro. 



Since the evidence above presented shows that the great body of 

 Adirondack anorthosite has a chilled gabbroid border which can 

 not possibly extend far out beyond the present exposure of the 

 anorthosite, and evidence below presented is distinctly against 

 existence of syenite or granite formed- as a differentiate between 

 the border facies and the typical anorthosite, it is clear that Bowen's 

 hypothesis of the origin of the anorthosite by the settling of 

 plagioclase crystals is untenable. There simply is nothing from 

 which they could have settled. The writer believes, therefore, that 

 it is out of the question to interpret the Adirondack igneous com- 

 plex as even in a general way a " sheetlike mass with syenite above 

 and anorthosite below " as required by Bowen's hypothesis. 



Relation of the syenite-granite to the Whiteface anorthosite. 

 According to Bowen, the syenite-granite and anorthosite are not 

 distinctly separate intrusives, but both formed as differentiates 

 from a single great body of intruded gabbroid magma. Gushing i 

 and the writer both believe the syenite-granite series to be dis-l 

 tinctly later, and the writer has found abundant evidence in sup- 

 port of this view in both the Lake Placid and Schroon Lake quad- 

 rangles. 



For the Long Lake quadrangle Gushing says 1 : " The field evi- 

 dence 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 the writer agrees, but he would further say 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 



. Geol., 25:507. 1917. 



