30 NEW YORK STATE MUSEUM 



of limestone caught up by the granite or syenite magma were con- 

 verted into amphibohte. 



Still another feature of interest is the occasional occurrence of 

 rapid transitions from syenitic to granitic material and vice versa, 

 giving rise to a kind of banded structure parallel to the foliation 

 but with the bands not at all sharply separated from each other. A 

 case in point is the freshly blasted ledge' on the road i^ miles south- 

 west of Long Lake village where a band of light-gray hornblende 

 granite (no. 42, table 5) 2^ feet wide passes by insensible grada- 

 tions on either side into a greenish gray pyroxene syenite (no. 41, 

 table 3). Such phenomena appear to be rather common in the 

 Adirondack region, many observations having been made in the 

 various quadrangles studied by the writer, and also in the Long 

 Lake and Elizabethtown-Port Henry quadrangles by Gushing and 

 Kemp respectively. For most part these banded structures are 

 believed to be a result of magmatic differentiation, but in some cases 

 it is probable that lenslike inclusions of Grenville rocks have been 

 more or less assimilated by the inclosing syenite or granite. 



Granite Porphyry 



Many times the pink or gray granites have suggestions of por- 

 phyritic texture, though there is but one small area of typical 

 granite porphyry which could be mapped as such. This is in 

 marked contrast with the relative prominence of such porphyry 

 within the Broadalbin, North Creek and Lake Pleasant quadrangles 

 mapped by the writer in the southeastern Adirondack region. 



The small area of granite .porphyry occupies about one-quarter 

 of a square mile in the vicinity of the Indian lake dam.- Some of 

 the rock has been used in the construction of the dam. There are 

 many good exposures, the best being at the dam and in the quarries 

 just east. Where fresh the rock is greenish gray, and where 

 weathered it is pinkish. The rock is always coarse grained and 

 rather gneissoid with the porphyritic texture usually well developed 

 though at times only poorly so. The feldspar phenocrysts are always 

 highly granulated. Locally biotitic shear zones occur. At times 

 some small Grenville gneiss inclusions may be seen. 



A thin section of the typical rock from near the dam shows the 

 following mineral percentages: orthoclase 10; microline 25; micro- 

 perthite 15; oligoclase to andesine 4; quartz 40; hornblende 2; 

 biotite I ; garnet 2 ; apatite ^ ; and a little zircon and magnetite. 



