DISTRIBUTION OF THE HYPERSTIIENE I\ BOILDERS. 33 



Hypersthene rock occupies, tlierefore, a triangular area, vvliicli is confined almost wholly to 

 the counter of Essex ; in fact, it would not be far frion the truth to consider all of this county 

 as composed of this material, excepting a belt a few miles in width along the shore of the lake, 

 which is gneiss and hornblende, and rocks of the Champlain group composing a part of the 

 transition class. The width of those rocks diminishes towards the norlli. In travelling west 

 from Port Henry to West Moriah, tiic rock becomes decidedly the hypersthene rock ui about 

 nine miles ; while from the lake at Westport, we fall upon the rock in three or four miles. 

 The passage from gneiss to hypersthene is marked by gradual changes. I have not been able 

 to find a line which marks, with any degree of distinctness, the place where the gneiss on tiie 

 one side ends, and hypersthene rock on the other commences. Viewed in the extremes, the 

 two rocks are widely different ; as they approach each other, the matc-rials of which each are 

 composed become so incorporated, that it is difficult to determine to which mass the interme- 

 diate portion belongs. 



The area which is occupied by this rock is the most mountainous district in the State, the 

 highest mountain ranking only second in height to any in the United States. 



Distribution of the Boulders of this Rock. — A rock whose locality may be so clearly de- 

 fined, and whose characters are so distinct, becomes one of the best rocks for observation on 

 the distribution of its boulders. I have, therefore, been careful to search for them, wherever my 

 pursuits have called me since the commencement of the survey. The result of my observations 

 is, tliat they are distributed in lines nearly south of the present rock in the county of Esse.t. 

 Thus they are found abundantly in the valley of the Mohawk ; at Amsterdam, they are very 

 common ; and still farther south, in the county of Orange, boulders of this rock attracted the 

 attention of mineralogists at a very early day, from their containing tolerable specimens of 

 opalescent feldspar. This county appears to be the limit of their distribution in this direction. 



Although the lines of distribution appear as stated, yet there are important and interesting 

 exceptions. On Lake Ontario, nearly west of the great hypersthene tract, boulders of hypers- 

 thene rock are quite common ; also on the St. Lawrence in the vicinity of Ogdensburgh. Li 

 relation to this line of boulders, however, it is not possible to determine satisfactorily their 

 origin ; and I am disposed to assign them an origin still farther north, rather than to the New- 

 York formation. There is a very good reason for this opinion ; it is this, there is no line of 

 boulders by which we can trace them to the region of this rock in Essex county, but they ap- 

 pear to be arranged on the lake and river in a direction north and south ; and though we may 

 find a few twenty miles east, they become more rare the nearer we approach the table land of 

 the Racket. Those which are found upon the lake and river, however, cannot be distinguished 

 from those in the valley of the Mohawk ; they belong to the same rock : but if they originated 

 in the Essex hjrpersthene rock, it seems very probable we should be able to trace them back 

 to their source.* 



* The diluvial mark..s upon the hypersthene rocli are nearly nurth and south, and an interesting example of them may be seen 

 in the valley of the Adirondack river ; they pass over one of the large beds of magnetic oxide of iron, and were finely exhibited 

 by the recent removal of the soil. This locality is nearly 2000 feet above Lake Champlain. 



Geol. 2d Dist. 5 



