THE UPPER SILURIAN PERIOD. 



119 



GENERALIZED SECTION OF THE UPPER SILURIAN STRATA 

 OF WALES AND SHROPSHIRE. 



Fig. 57- 



Base of Old Red Sand- 

 stone. 



~- Pile-stones. 



Upper Ludlow Rock. 

 Aymestry Limestone. 



Lower Ludlow Rock. 

 Wenlock Limestone. 



f Wenlock Shale (Den- 

 ] bighshire Flags and 

 [ Grits of North Wales). 



Woolhope Limestone. 

 Tarannon Shales. 

 May Hill Sandstone. 



Llandovery Rocks. 



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 Hcldcrbcrg 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 

 which are divided into several zones, capable of recognition by 

 their fossils, and known by local names (Tentaculite Lime- 

 stone, Water-lime, Lower Pentamerus Limestone, Delthyris 



