REPORT OF THE DIRECTOR I917 II9 



Granites in the Doubtful Gneisses 



In the doubtful gneisses north of Saranac Lake village several 

 exposures were found, which in appearance suggest a true granite 

 gneiss. It is a question whether they are immediately associated 

 with the Grenville or are a true granite older than the later great 

 series of igneous rocks, but distinct from the old sedimentaries. 

 In view of the wide difference of opinion and lack of sufficient data, 

 the writer is compelled to take refuge in a noncommittal mapping; 

 grouping all such gneisses and granite-gneisses together, and indi- 

 cating them by a uniform convention. 



The Pegmatites 



As the surface of the molten rocks (magma) began to solidify, 

 shrinkage cracks occurred due to the contraction on cooling, and 

 these allowed the still molten interior to escape upwards in the 

 form of various gases and liquids. These aqueous vapors filled the 

 cracks and congealed, solidifying at comparatively low temperatures, 

 forming bands or dikes of highly siliceous minerals, such as the 

 feldspars and quartz. Each of- the different igneous rocks above 

 mentioned shows such dikes or " pegmatites," although the granites 

 and more siliceous rocks are more productive of them. One such 

 pegmatite cutting the anorthosite was observed directly in front of 

 the Lake Clear Inn near the water line. It is 3 J inches wide, runs 

 west-northwest, and exhibits the characteristic coarse crystals of 

 orthoclase feldspar (probably the variety known as microcline) and 

 quartz. Pegmatites of the syenites and gabbros abound in the 

 Adirondacks. Several syenitic pegmatites cutting the anorthosite 

 were found a little south of Mountain pond northwest of Paul 

 Smith's, and are indicated on the map. 



Contact Rocks 



Igneous Contacts 



When the anorthosite, syenite and gabbro, in forcing their way 

 up to and into the surface rocks encountered other igneous bodies 

 already solid and cool, the effect of the chill upon them is very 

 marked and may be observed for a mile or two on both sides of the 

 line of contact. The border phase of the anorthosite is shown by 

 an increasing amount of the darker minerals (pyroxenes and garnets) 

 at the expense of the labradorite, thus approaching the gabbro in 

 composition and giving rise to " anorthosite-gabbro." The syenite 

 likewise passes through changes in composition, becoming darker in 



