ON THE EXHAUSTION OF OUE COAL. 295 
coal, and of extending those workings both horizontally and 
vertically. 
1 . Geological investigation has shown ns that coal belongs 
to a special group of rocks, which has been named the Car- 
boniferous group. This formation assumes, even in different 
parts of those islands, several peculiar variations, which 
clearly prove that they do not belong to the same age ; that 
although the same general conditions of moisture and heat 
necessary for coal formation have prevailed, the mechanical 
phenomena of the transportation and deposition of car- 
bonaceous and earthy matter have greatly varied. For 
example, in Western England and South Whales we find three 
well-defined divisions in the Carboniferous group : — 
{ Strata of shale, sandstone, and grit, from 600 to 
12,000 feet thick, with occasional seams of 
coal. 
( A quartzose sandstone, often a conglomerate, 
2. Millstone Grit < with beds of shale, altogether more than 
( 600 feet thick. 
3. Mountain Limestone 
( A calcareous rock, of marine origin, sometimes 
( 900 feet thick, devoid of coal. 
In the North of England, beds of limestone are found in the 
Millstone Grit, and even a few seams of coal, and in some 
parts of the Scotch coal-field we find an intercalation of the 
marine limestones, with sandstones and shale, containing coal. 
That coal has been formed from vegetable matter, no 
longer admits of any doubt. The processes by which 
the ancient forests, the peat - like formations of semi- 
tropical swamps, or the plants of marine growth have been 
converted into coal, cannot be said to be clearly de- 
termined. This much, however, must be admitted. That 
there must have been extensive tracts of undrained land 
upon which the vegetation found in the coal measures must 
have grown. That inland seas, or lakes, or waters, under 
some conditions compelling repose, must have existed, or the 
shales, the clays, and the coal, could not have been deposited. 
The Mountain Limestone indicates marine conditions, ana- 
logous to those which now prevail amongst the Coral Islands of 
the Pacific Ocean. Desiring to avoid every controversial point, I 
am with intention especially general ; the only conditions which 
concern the question under consideration, being, that the coal 
epoch proper was from the termination of that period which we 
distinguish as the Old Red- Sandstone age, and the commence- 
ment of that which belongs to the New Red- Sandstone time. In 
other words, no coal must be expected below or in the Old 
Red- Sandstone rocks, nor must we imagine that true old coal 
