TRENTON LIMESTONE. 61 
TRENTON LIMESTONE. 
This limestone is more important than either of the preceding, in geographical extent, 
thickness, and number of fossils. It usually succeeds the Black-river limestone, or, in its 
absence, the Birdseye limestone, in thin shaly layers, or in compact strata separated by 
shaly lamine. In a few instances the rock is concretionary in its lower portions, a large 
proportion of shaly matter being intermingled. The rock is for the most part thinbedded, 
of a dark blue or black color. Towards the higher part of the mass we usually find some 
thickbedded grey subcrystalline strata, bearing quite a different aspect from the rock below. 
Tracing the rock upwards from this point, we find a gradual increase of shaly matter, 
until finally the whole mass becomes a shaly limestone, thence passing into the succeeding 
black shale by imperceptible gradations. 
The rock is highly fossiliferous throughout, some localities furnishing more than others, 
and offering better facilities for procuring them. The number of species figured show how 
prolific it has proved, even under a very imperfect investigation, Perhaps none of the 
higher groups furnish a greater number of species in the same extent of thickness and 
surface exposed, than does this rock. 
In describing the rocks of New-York in the Geological Reports, it was found convenient 
to separate the Trenton limestone from the Utica slate, and the succeeding shales and 
sandstones of the Hudson-river group. The same distinction has been kept up in the present 
Report, though a more thorough investigation of the palzozoic features of these different 
rocks has proved that many of the fossils are common to the Trenton limestone and the 
Hudson-river group, while a few species of the limestone appear in the Utica slate. Among 
the trilobites, we may mention the Calymene Blumenbachi, which abounds in the Trenton 
limestone, is rare in the Utica slate, and more frequent in the Hudson-river group. The 
Trinucleus is common in the Trenton, abundant in the Hudson-river group, though rarely 
seen in the Utica slate. The Triarthrus Becki abounds in the Utica slate, and is rare both 
above and below, though occurring in both situations. 
Nearly all the species of the Bracutopopa of the Trenton limestone reaéppear in the 
Hudson-river group, though rarely seen in the intervening slate. There are, at the same 
time, species in the Hudson-river which are unknown in the Trenton limestone, and vice 
versa, though their paleozoic relations are very intimate throughout. 
From all these facts, I am disposed to unite the whole as one group, still retaining the 
subdivisions for convenience of reference. This seems the more desirable, since at the west, 
particularly in Ohio and Indiana, the augmentation of calcareous matter has made it im- 
possible to draw any line of demarcation which shall correspond with the three divisions 
so obviously marked by their lithological characters in New-York. 
. 
