GEOLOGY OF LAKE PLEASANT QUADRANGLE 1 5 



mineralogical variations of the normal syenite are clearly brought 

 out in the table on page i6. 



The color, textural, and structural variations are scarcely less 

 notable. Where fresh, the normal syenite shows the dark greenish- 

 gray color so common throughout the Adirondacks, and the 

 weathered surface, from a fraction of an inch to 2 or 3 inches 

 thick, shows a light brown color. Ledges in the woods often are 

 light gray on the immediate surface, probably due to the action of 

 decomposing organic matter upon the iron compounds. Such light 

 gray to almost white surfaces are particularly well exhibited on the 

 top of Cathead mountain recently burned over. 



The normal syenite is typically medium grained though at times 

 it varies to fine grained, while again it becomes moderately coarse 

 grained to even porphyritic. Granulation is nearly always shown, 

 sometimes to a high degree especially as regards the quartz and 

 feldspar. 



The rock is always more or less foliated. In some ledges the 

 gneissic structure is so faintly developed as to be made out with 

 difficulty if at all. In other places the foliated structure is so 

 thoroughly developed that the rock presents an almost schistose 

 appearance. Such a high degree of foliation is frequently asso- 

 ciated with and parallel to shear zones along which biotite has 

 developed at the expense of pyroxene or hornblende. Such rocks 

 often split readily into thin layers. Among other good examples 

 is the ledge in Elbow creek at the mouth of the gorge a mile north 

 of Wells. The finer grained syenites are mostly associated with 

 such shearing which has produced excessive granulation. Most 

 commonly the gneissic structure is so developed as to give the rock 

 a distinctly streaked but not banded appearance due to a drawing 

 out of the dark minerals into crude parallelism. 



That the syenite is younger than the Grenville is proved by the 

 presence of distinct Grenville gneiss inclusions such as near the 

 summit of Fish mountain; just southwest of the summit of Mount 

 Dunham; along Elbow creek between Elbow mountain and Mount 

 Rouge; one-half of a mile south of the mouth of Colombe brook; 

 just north of Jacksons seat; and at many other places. The normal 

 syenite, however, is apparently freer from inclusions than its more 

 basic or acidic phases or than the granite. In certain of the mixed 

 gneiss areas the syenite is seen to have literally cut to pieces the 

 Grenville. 



