NIAGARA AND LOWER HELDERBERG GROUPS. 129 



At this point the Niagara group is separated from the con- 

 tinuation of the Lower Helderberg group by strata of more 

 than 1,000 feet in thickness. 



Everywhere throughout New York the Lower Helderberg 

 group is underlaid by the water-lime formation ; and the same 

 is true in New Jersey, Pennsylvania, Maryland and Virginia ; 

 and everywhere throughout New York and Canada West, and 

 in Wisconsin and Iowa the water-lime formation lies above 

 the Niagara group, or its representative, the Coralline lime- 

 stone.* In no case do these two formations come together, 

 except where the water-lime formation is absent. 



Certainly these formations are widely enough separated to 

 constitute distinct groups over the areas named. 



It is suggested in the paper cited that the difference between 

 the fossils of the Lower Helderberg group in eastern New York, 

 and those of the Niagara group in the central and western 

 part of the State, is due to "a change in the lithological char- 

 acter of the beds in their eastern extension." 



In the western part of the State, the Niagara group is com- 

 posed of calcareous shales and dolomites. The Lower Helder- 

 berg group in the eastern part of the State consists, in its lower 

 part, of thick and thin bedded dark or black limestones, with 

 shaly partings, and sometimes with thicker intercalated shaly 

 layers ; to these succeed the heavy bedded limestone with 

 Pentamerus galealus, which by the intercalation of shaly 

 matter becomes thin bedded, and passes by almost insensible 

 gradations into the " Shaly Limestone," and finally to a silico- 

 calcareous shale.f The higher member, in many localities, is 

 the thin bedded Upper Pentamerus limestone, while at 

 Becraft' s mountain and in the Helderberg the upper member is 

 a heavy-bedded encrinal limestone, sometimes known as the 

 Scutella limestone, from the presence of great numbers of the 

 bases of Aspidocrinus. The shales of the Niagara group and 

 their contained bands of limestone, which are the most highly 

 fossiliferous portion of the group in New York, are not dolo- 



* It is true that over a considerable part of the lake region, the water-lime and 

 Onondaga salt group have been eroded from above the Niagara formation ; the 

 place of these softer formations being occupied by the lakes. See Foster and 

 Whitney's Report on the Lake Superior Land District. 



+ The physical aspect of this portion of the group is preserved in the " siliceous 

 li/mestones " of this age in the south-west. 



17 



