28 SKETCH OF THE GEOLOGY OF DERBYSHIRE. 
state of preservation ;* and it is among these gritstones that we find 
first traces of beds of coal. Beds of coal are formed of masses of 
vegetable matter, which being probably diffused over wide areas of 
water, to the bottom of which it gradually sank, and undergoing a 
kind of process similar to fermentation, in which probably heat was 
generated ; and being also compressed by the weight of other mat- 
ters, by which it got gradually covered, has at length been converted 
into coal. For the equal diffusion of sometimes very thin beds of 
this vegetable matter over extremely wide areas, some length of time 
must certainly be allowed. An equal thickness of shale would take, 
perhaps, less time, but even that we see to have been slowly deposited, 
on account of the thinness of its lamine. Gritstones we may judge 
to have been slowly or rapidly accumulated, according to the fine- 
ness or coarseness of the material, their deposition, in every case, 
being probably more rapid than the shale. Of these three things, 
namely, shale, gritstone, and coal, the whole of the coal measures 
are made up, in indefinite and almost innumerable alternations, to 
the thickness of 2000 feet. 
The time required for any regular system of operations by which 
such a succession of events could have been brought about, could not 
but have been considerable. Such must have been the means by which 
the carboniferous system of rocks was produced, and, even under the 
most favourable circumstances that we can conceive, it must have 
been the work of many, many ages. But we have made no allow- 
ance for periods of rest, during which nothing was deposited. 
There must have been some interval between each successive bed, 
though of how great duration we cannot tell. If we look, too, at 
the change produced during the formation of the whole, our idea of 
the time occupied will be still farther enlarged. We find a tran- 
quil and deep sea to have been gradually filled up, or at least so 
altered as for currents to drift along its buttom, and for the animals 
contained in it to have become extinct, and at length for creatures 
belonging to fresh water to exist over its area.t The physical geo- 
graphy of this part of the globe must have undergone some impor- 
* The great fossil trees—the occurrence of which in an inclined position, 
and apparently piercing through many beds of sandstone, seems to puzzle so 
much a certain class of geologists (by courtesy so called), whose minds are 
more fitted to the discovery of petty difficulties than the reception of general 
truths founded on large bodies of evidence—are always found in these irre- 
gularly bedded and comparatively quickly formed gritstones, 
+ This is shown in the “ muscle bands” of the coal measures. 
