184 
GEOLOGY OF THE SECOND DISTRICT. 
slate. The slate, both above and below Glen’s-Falls, contains 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 miles above the falls, it is less than fifteen feet; at Sandy-Hill, three 
miles below, it is thirty or thirty-five. At neither of these places, therefore, does it appear in 
its full thickness, either from having thinned 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 are confined to one 
or two feet in breadtli. 
The most interesting dyke which fell under 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 connected 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 
cooling, 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 graptolites, we find a very small neat bivalve, which appears to be a Posidonia; it is scarcely more than 
a line in diameter, and is extremely thin and very delicately striated. 
