130 
THE GEOLOGIST. 
of different kinds— e.^., sandstones, limestones, and argillaceous beds 
called rocks. The different strata have been formed step by step, 
layer upon layer. Each stratum has its distinct position and age in 
relation to other strata. The relative age and, consequently, position of 
a rock can be distinguished by mineral structure, fossU contents, or 
both. 
Again, by way of still more clearly establishing our primary position, 
let the following table represent some of the strata above and below 
the general coal deposit : — 
Gboup. Eocks. 
r Upper ■) 
Oolitic j Middle V 
( Lower j 
Liassic 
Triassio i Keuper Marls ) 
±riassio [ i^-gw Eed Sandstone ) 
T> . ( Masrnesian Limestone ) 
Permian j j^^-^^ | 
Coal Measures 
Mountain Limestone. 
Old Red Sandstone. 
Silurian Strata. 
Now this partial table of British strata represents the coal measures as 
being overlaid by the JSTew Red Sandstone deposits, and underlaid 
by the Mountain Limestone and Old Red Sandstone, which, though a 
general, is by no means the invariable rule in coal deposits, as sometimes 
the Mountain Limestone is absent, and the coal measures rest upon the old 
red system, as in great part of the Shropshire coal-field. In others even 
the Old Red is absent, and they repose directly upon the Silurian rocks, 
as in most part of the Staffordshire basin. Still, inasmuch as the 
Mountain Limestone is the normal stratum upon which coal rests, we 
may pronounce it as a fact that this limestone is older than the coal 
itself ; and so confidently assert that boring for coal wiU be a fruitless 
operation if attempted from Mountain Limestone, or any other under ■ 
lying formation. The reason, therefore, why coal may rest upon 
any other rock older than itself arises simply from the absence of inter- 
mediate strata, which so far has interfered with the sequence but not 
the order of super-position of coal and the deposits below : and the 
Peobable tuickness. 
1,800 feet. 
1,200 feet. 
1,500 feet. 
500 feet. 
5,000 feet. 
