FORMATIONS OF ORDOVICIAN AGE. 49 
nearly all indicate a lower Trenton or upper Black River horizon, and 
it may doubtless be stated with safety that the heavier beds of lime- 
stone in the formation occupy this horizon. In no other locality in 
the State, aside from Jacksonburg, so far as has been observed, have 
the higher beds of the formation been represented by limestones. In 
most cases these beds seem to be calcareous shales, which rarely oui- 
crop on the surface, and which grade imperceptibly from the lime- 
stones below into the typical Hudson river slates above. Sometimes, 
as on the Pequest river near Belvidere, the deposition of the cal- 
careous shales seems to have begun earlier than usual, even as early 
as the lower Trenton horizon. 
The numerous field observations by Dr. Ktimmel indicate that the 
average thickness of the formation as a whole, from the contact with 
the Kittatinny limestone below to the transition into the typical Hud- 
son River slates, varies from 135 to 150 feet in Sussex county, but 
increases to 300 feet or more at the Delaware river; the relative thick- 
ness of the limestones below and the calcareous shales above is, how- 
ever, probably quite variable—in some localities, as at Jacksonburg, 
the limestones extending through nearly the entire thickness of the 
formation, and again, being very thin, as in the neighborhood of 
Belvidere. 
HUDSON RIVER SLATES. 
The Hudson River slates constitute the surface formation over a 
large portion of the Kittatinny valley and occur, also, in some of the 
outlying paleozoic areas surrounded by the crystalline rocks. The 
formation rests conformably upon the Trenton limestone and is not 
sharply separated from it, there being a gradual gradation from the 
limestone of the lower formation, through calcareous shales, to the 
typical slates and arenaceous beds of the Hudson River formation. 
Because of the great amount of folding and faulting in the region 
the thickness of the formation has not been determined, but it may 
probably be safely stated to be several thousands of feet. Wa 
The name Hudson River is here used strictly as a formation name, 
these beds evidently being continuous with the great mass of contorted 
slates along the Hudson river in New York State, to which this name 
was first applied. The transition from the calcareous to the clay and 
sand sedimentation seems to have taken place earlier in the ancient 
Ordovician ocean, towards its eastern shore, in the region which is 
now Eastern New York and Northwestern New Jersey, and probably 
4 
