264 POTSDAM SANDSTONE. 



POTSDAM SANDSTONE. 



It was stated by Prof. Adams in his second Report, that this rock did not occur in Ver- 

 mont. A careful examination, however, has satisfied us, that to a limited extent it may be 

 found. Its lithological characters vary very little. In consequence of a metamorphic 

 action, we reckon three varieties : 



1. PURE SILICIOUS SANDSTONE. 



2. HORNBLENDE SCHIST. 



3. GNEISS. 



The first of the three is what is universally known as the Potsdam sandstone, a hard, 

 compact, thick-bedded sandstone, and perfectly homogeneous in structure, unless meta- 

 morphosed. Its color is generally white, in the eastern part of North America ; it being 

 red upon the shores of Lake Superior. The common variety may be seen in great per- 

 fection upon the hill overlooking the town of Whitehall upon the east. Nos. j 2 and 



in the Cabinet, represent the unchanged variety from Whitehall. No. j^ is the same 

 variety from West Haven of a red color. In small quantities the former occurs in Ver- 

 mont, in West Haven, and upon Mount Independence, in Orwell. 



The second variety (Nos. jf^ and jf^) cannot be distinguished from the hornblende 

 schist which occurs among the azoic rocks of Vermont and New York. The specimens 

 were obtained from the extreme south part of West Haven, in the upper part of the 

 formation. It is not found in distinct beds, but is scattered in patches through the hill. 



The third variety very closely resembles the laurentian gneiss. It seems to pass into 

 it by insensible gradations. The specimens obtained (Nos. j 9 to ), are from the south- 

 west part of West Haven. All the constituents of this rock are very small, and occasion- 

 ally the feldspar or the mica may be wanting. These hornblende schists and gneissrocks 

 indicate that a change has passed over a part of the Potsdam sandstone, analogous to 

 that metamorphism that has so obscured the rocks of Eastern Vermont ; for such rocks 

 as gneiss, etc., are not formed except by the crystallization of the constituent minerals 

 after the accumulation of the sediments. 



Associated with these crystalline schists are veins of granite ; whose feldspar is Lab- 

 radorite, (Nos. jf, and ffg.) This mineral is mostly confined to rocks below the Silurian 

 system ; and in West Haven it extends only a few feet into the base of the silurian, and 

 that in small veins from three to ten inches in width. 



STRIKE AND DIP. 



Locality. Strike. Dtp. Olserver. 



Whitehall, N. Y., N.20E., 10 E., Prof. Mather. 



West Haven, southwest part, N. 10 E., 8 E., C. H. H. and E. H., jr. 



West Haven, west part, East of north, 4-5 E., C. H. H. and A. D. H. 



Orwell, Mt. Independence, East of north, 4 E., C. H. H. 



Ticonderoga, N. Y., 10 N.E., C. H. H. 



Shoreham, east part, N. 10 E., 14" E., C. H. H. 



The unconformability of the dip of this rock to the Laurentian beneath, may be seen at 

 the extreme southern point of West Haven. Upon the lake opposite to the termination of 



