l8 NEW YORK STATE MUSEUM 



dark, basic Grenville gneiss by the syenite magma and also that a 

 much larger mass of basic syenite at the Hooper mine near 

 Thirteenth lake in Warren county is almost certainly due to mag- 

 matic assimilation of basic gneiss. Specimens from these two 

 localities show almost exactly the same characteristics as the typical 

 basic syenites from the Lake Pleasant quadrangle. 



The Speculator-Lookout mountain area of so-called basic syenite 

 is the largest within the Lake Pleasant quadrangle. Numbers 12, 

 13 and 17 of the table on page 16 represent the most basic and 

 least syenitic facies of the area. While all the rocks are distinctly 

 basic, nevertheless considerable variations in mineral content occur. 

 Thus near the west end of the area the rock is rather coarse grained 

 and gabbroic-looking, sometimes with many garnets and sometimes 

 without them. In the creek just east of Sand lake, the rock looks 

 much like certain garnet-bearing, border phases of anorthosite 

 described by both Gushing and Kemp. The garnets do not appear 

 to be of secondary origin. On the mountain a mile southeast of 

 Sand lake, the garnets disappear and the rock passes over into 

 a good hornblende syenite. The Speculator mountain mass looks 

 rather syenitic with garnets in the rock along Sucker brook and 

 considerable biotite and few garnets at the mountain summit. 



There is considerable evidence that this Speculator-Lookout 

 mountain mass has been produced by magmatic assimilation where 

 dark, hornblende-feldspar-garnet Grenville gneisses were incor- 

 porated into the normal syenite magma. This suggestion is borne 

 out by the actual presence of a considerable inclusion (see map) of 

 such rock one-half of a mile southeast of the southern end of Lake 

 Pleasant. In thin-section this inclusion contains the following 

 mineral percentages : hornblende 40 ; oligoclase to labradorite 30 ; 

 hypersthene 18; garnet 10; and magnetite and pyrite 2. The borders 

 of the inclusion appear to have been well fused by the enveloping 

 magma and in places this assimilation product greatly resembles 

 the typical basic phase of syenite just to the west. This inclusion 

 doubtless represents one of the last fragments of dark gneiss to have 

 been engulfed by the syenite magma when the temperature had 

 become too low for complete melting and assimilation of the frag- 

 ment. If many such masses of dark gneiss were actually enveloped 

 by the normal syenite magma and fully assimilated, the origin of 

 this large area of basic syenite finds a ready explanation. That 

 this magma did break through and rather thoroughly assimilate con- 

 siderable quantities of such basic Grenville rocks are strongly sug- 



