GEOLOGY OF THE SCHROON LAKE QUADRANGLE 2Q 



In the Schroon Lake quadrangle a number of clearly defined 

 dikes of granitic syenite and granite were observed in the anortho- 

 site. One of these is well shown by the road i l / 2 miles west of 

 Boreas river where a dike of granite 5 feet wide cuts rather femic 

 Whiteface anorthosite without very sharp contacts. Another is 

 a dike of typical pinkish gray granite 25 feet wide I mile west of 

 the western summit of Sand Pond mountain. It sharply cuts 

 Whiteface anorthosite which lies near, and closely resembles 

 Marcy anorthosite. Both dikes just mentioned are quite certainly 

 off-shoots from large bodies of typical granite which, in a general 

 way, cut into the marginal portion of the great body of anortho- 

 site. A wide dike of gray granite cuts Marcy anorthosite just 

 north of the summit of Texas ridge (see map). The granite is 

 very gneissoid with highly flattened quartz and feldspar crystals. 

 The small mass of Whiteface anorthosite on the side of Beech hill 

 (see map) is cut by a number of narrow dikes of granitic syenite 

 and granite which are doubtless off-shoots from the large sur- 

 rounding body of syenite-granite. Contacts between these dikes 

 and the anorthosite are usually not very sharp. 



Also in the Schroon Lake quadrangle many dikes or narrow 

 intrusive bodies of syenite and granite were observed in the areas 

 of anorthosite and syenite-granite mixed rocks, and to some 

 extent in the areas of Keene gneiss. Some of these are referred 

 to below. The evidence, therefore, from the dikes, that the syenite- 

 granite series is distinctly younger than the anorthosite series, is 

 very strong. 



Broad intrusive tongues of syenite and granite in anorthosite. 

 Broad tongues of syenite and granite extending, in a number of 

 places for miles, into the great body of anorthosite furnish per- 

 haps even more impressive evidence than the dikes that the syenite- 

 granite series is really younger than the anorthosite. 



Cushing's Long Lake geologic map shows an intrusive tongue 

 of syenite from I to 3 miles wide cutting into the anorthosite for 

 a distance of 2 miles. 



The writer's Lake Placid geologic map shows a fine example of 

 a tongue of syenite-granite with a maximum width of 1^2 miles 

 cutting Whiteface anorthosite across a portion of Wilmington 

 mountain. A great body of syenite-granite from I to 6 miles wide 

 extends into the anorthosite for 13 miles across the Lake Placid 

 quadrangle, and thence for an unknown distance into the Mount 

 Marcy quadrangle on the south. 



