THE. UPPER SILURIAN PERIOD. EZ 
GENERALISED SECTION OF THE UPPER SILURIAN STRATA 
OF WALES AND SHROPSHIRE. 
Fig. 57. 
Cie Ose, wie. o JO, © 
i { Base of Old Red Sand- 
St a a Ee 
(stone: 
im 
2 Tile-stones. 
4 
oO 
2 Upper Ludlow Rock. 
a Aymestry Limestone. 
4 Lower Ludlow Rock. 
= Wenlock Limestone. 
e 
O Wenlock Shale (Denbigh- 
4 shire Flags and Grits of 
Q North Wales). 
A 
SI 
= Woolhope Limestone. 
Tarannon Shales. 
so a ee “fs 3 pay : tee = ; 
See Ae St May Hill Sandstone. 
i ee ay Pea er = 
er °o . 
Be Ported he mea ee. 
May- 
Hit 
GROUP 
Llandovery Rocks. 
(3) Magara 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 
