110 
GEOLOGY OF THE SECOND DISTRICT. 
38. 
Fig. 38 illustrates the great regularity of this anastomosis, as it appears upon the surface 
of one of the layers of this rock; though it is rare to find so much regularity, yet the inter¬ 
communication appears to have been always more free and regular upon the surface of a layer 
than during its formation. 
A most interesting fact is furnished at numerous places where this rock occurs, but par¬ 
ticularly at the village of Fort-Plain in the valley of the Mohawk ; it is the sudden extinction 
of this animal. Thus, on the immediate plane where the Trenton limestone is deposited, 
this fossil disappears, and no trace is to be seen of it in the succeeding rock; while up to the 
commencement of the Trenton limestone, it appears to have been in full vigor. The fact that 
it does not occur in the drab colored layers, though in the midst of the birdseye, throws some 
light upon the cause of its sudden extinction: they are impure limestones, and the matter 
deposited with the particles of lime exerted an unfavorable influence upon it. So the Tren¬ 
ton limestone, being composed of aluminous and other earthy matter in part, formed a medium 
which became unsuitable to their peculiar mode of living. 
The thickness of the birdseye is about 30 feet; but like other limestones in this group, it 
thins out remarkably towards the south. At Essex on Lake Champlain, it appears to be 
entirely wanting, the Trenton resting directly upon the Chazy limestone. At some other 
places, it is very thin; but notwithstanding this, it preserves its lithological characters in 
perfection. 
Black Marble of Isle La Motte. 
Between the Birdseye and Trenton limestone, there is an important mass whose relations 
have not been well understood. It is unimportant, so far as its thickness is concerned; yet, 
as it is largely employed for furnishing one of the finest marbles in this country, it becomes 
necessary to notice it as a distinct rock. It is a black, finely granular mass, susceptible of a 
high polish. At the Isle La Motte, it forms as it were but one thick bed, or a single bed 
comprehends all the workable material for marble. There are planes of deposition separating 
it into several layers, but they are quite obscure. The mass is about twelve feet thick. A 
similar mass, occupying the same position, has been wrought many years at Glen’s Falls. It 
is not so perfectly compacted into one thick layer, as at Isle La Motte; but it presents the 
same fine even texture, and forms a marble equal in value. At Watertown in Jefferson 
