230 THE GEOLOGIST. 



of different kinds— 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, fossil 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 : — 



Qroup. Eocks. Puoba-ble thickness. 



r Upper ■) 

 Oolitic ] Middle [- 1,800 feet. 



( Lower j 

 Liassic 1,200 feet. 



Coal Measures 5,000 feet. 



Mountain Limestone. 

 Old Eed Sandstone. 

 Silurian Strata. 



ISTow this partial table of British strata represents the coal measures as 

 being overlaid by the l^ew Eed Sandstone deposits, and underlaid 

 by the Mountain Limestone and Old Eed 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 Eed is absent, and they repose directly upon the Silurian rocks, 

 as in most part af 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 will 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 



