184 GEOLOGY OF THE SECOND DISTRICT. 



slate. The slale, both above and below Glen's-Falls, contams in abundance the graptolite,* 

 which so nearly resembles a small fern leaf, that it is usually so considered by those little 

 acquainted with fossils. 



This is the highest rock which geologically appears in Warren county ; but it has an 

 extremely limited extent, being found only along the banks of the Hudson in the vicinity of 

 Glen's-Falls ; or if it extends for some distance, it is concealed beneath the deep sand of the 

 adjacent region. 



The thickness of the slate at the south side of the river at Glen's-Falls, is about twenty 

 feet ; at the locality two mdcs above the falls, it is less than fifteen feet ; at Sandy-Hill, tliree 

 miles below, it is thirty or thirty-five. At neither of these places, therefore, does it appear in 

 its full thickness, either from having thimied out, or from having been washed away. 



Trap, or Igneous Rocks. 



So limited are the rocks under this denomination, that all the important facts in relation to 

 them may be comprised in a few words. 



In the first place, there are no extensive masses of igneous rocks in the county : they are 

 all confined to narrow dykes, which impart no very distinct features to the country ; they are 

 entirely local in their effects, and their influences as it regards other rocks arc confined to one 

 or two feel in breadtli. 



The most interesting dyke which fell r.nder my notice, is at Johnsburgh, near the residence 

 of Mr. Rosevelt. It is about two feet wide, and runs N. 55= E. It is in the usual form of 

 trap, consisting of fine particles compacted together, with only a slight tendency to form in 

 the mass a columnar or tabular structure. It is black or dark green, weathering to a brown 

 as usual. 



The most interesting fact comiected with this dyke, and it is the only one of the kind which 

 has fallen under my notice, is the formation of obsidian, at the line of contact with the gneiss 

 in which it is embraced. The obsidian, or the part which so strongly resembles it, is per- 

 fectly compact, with a vitreous lustre and a bluish black color, and a conchoidal fracture. It 

 is only about one inch wide on both sides of the dyke. It is to be considered as a part of the 

 dyke, which for some cause was more perfectly pure ; and in consequence of more sudden 

 cooUng, from contact with the rock, assumed the more vitreous form of obsidian. Those 

 parts whose temperature was reduced slowly, assumed the stony aspect, with a slightly 

 granular or crystalline structure, there being time for the particles to arrange themselves with 

 some regularity. 



Dykes of trap cross the road between Glen's-Falls and Caldwell, near the place where it 

 is supposed Col. Williams fell in battle with the Indians. Their direction is N. 40° E. 



" Accompanying the graptolitcs, we tine! a very small neat bivalve, which appears to he a Posidonia; it is scarcely more than 

 a line in ('.iametcr, and is extremely thin and verj' delicately striated. 



