FORMATIONS OF ORDOVICIAN AGE. 17 
always be detected between the two. The average thickness of this 
formation is from 135 to 150 feet, and although it is a limestone, it 
is very different in character from the subjacent Kittatinny limestone. 
Chemically it is a nearly pure calcium carbonate, while the Kittatinny 
limestone is almost always highly magnesian. It becomes, however, an 
argillaceous limestone in the transition beds to the overlying shale. Itis 
usually much darker in color than the Kittatinny limestone and almost 
always contains fossils, although they are frequently fragmentary and 
poorly preserved. The formation was described by Professor Cook* as 
the fossiliferous limestone and its Trenton age has long been recog- 
nized. It rests unconformably upon the Kittatinny limestone, the two 
formations being separated by an erosion interval, and in its lower beds 
it is frequently conglomeritic, the pebbles being from the underlying 
Kittatinny limestone. In some localities, where it is not conglomeritic 
at the base, its lower portion consists of fine, clastic material, ap- 
parently derived from the underlying Kittatinny limestone. In many 
localities the formation is apparently absent, the Hudson River slates 
resting directly against the Kittatinny limestone, but in all these cases 
its absence is due to overthrust faulting. 
A large number of fossil species have been recognized in the forma- 
tion at its various localities in the State, and a critical study of its 
faunas shows that it is not strictly equivalent to the typical Trenton 
limestone of New York, but that it also includes somewhat older beds 
of the age of the Black River limestone. The entire formation, how- 
ever, in New Jersey constitutes a single stratigraphic unit, and this 
unit may be considered as being the equivalent of the Black River and 
Trenton limestones of New York, although the top of the formation 
in New Jersey is doubtless lower down in the series than the summit 
of the typical New York Trenton limestone. 
Several distinct faunal zones have been recognized in the formation. 
The outcrops, however, are usually more or less isolated knolls, and 
are usually so fully covered with debris that fossils can be collected 
only from loose fragments of the limestone on the surface. It is also 
rare to find more than a single faunal zone in any one outcrop. Under 
these circumstances the determination of the exact succession of the 
faunal zones has been a problem of some difficulty. The locality 
selected for a detailed investigation of the succession of fossil faunas 
in the formation is a hill, 580 + feet in elevation, on the property 
* Geol. N. J., 1868, p. 131. 
