TKENTON 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 laminae. 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 £ispect 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 oflTering 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 palaeozoic 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 Blumenhachii, 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 Beckii abounds in the Utica slate, and is rare both 

 above and below, though occurring in both situations. 



Nearly all the species of the Brachiopoda of the Trenton limestone reappear 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 palaeozoic 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 

 80 obviously marked by their lithological characters in New- York. 



