THE UPPER SILURIAN PERIOD. 



117 



GENERALISED SECTION OF THE UPPER SILURIAN STRATA 

 OF WALES AND SHROPSHIRE. 



Fig. 57- 



J Base of Old Red Sand- 

 ( stone. 



Tile-stones. 



JH Upper Ludlow Rock. 



Aymestry Limestone. 



Lower Ludlow Rock. 



Wenlock Limestone. 



Wenlock Shale (Denbigh- 

 shire Flags and Grits of 

 North Wales). 



Woolhope Limestone. 



Tarannon Shales. 

 - May Hill Sandstone. 



-- Llandovery Rocks. 



(3) Niagara Group. This group consists typically of a 

 series of argillaceous beds (" Niagara Shale ") capped by 

 limestones ("Niagara Limestone"); and the name of the 

 group is derived from the fact that it is over limestones of this 

 age that the Niagara river is precipitated to form the great 

 Falls. In places the Niagara group is wholly calcareous, 

 and it is continued upwards into a series of marls and sand- 

 stones, with beds of salt and masses of gypsum (the " Salina 

 Group"), or into a series of magnesian limestones (" Guelph 

 Limestones "). The Niagara group, as a whole, corresponds 

 unequivocally with the Wenlock group of Britain. 



(4) Lower Helderberg Group. The Upper Silurian period 

 in North America was terminated by the deposition of a series 

 of calcareous beds, which derive the name of " Lower Helder- 

 berg" from the Helderberg mountains, south of Albany, and 



