SKETCH OF THE GEOLOGY OF DERBYSHIRE. 27 
immediately above the surface of the toadstone, we frequently find 
numerous shells, as evidently as elsewhere in the position in which 
they lived and died ; and the limestone that lies upon the toadstone 
does not differ from other beds, and therefore the lava must have 
time to grow cool before any of the superior beds were deposited 
upon it. As we should expect, we find the upper beds of the lime- 
stone, or those which were deposited when the sea became shallower, 
more abounding in animal remains than the lower ; for we know 
that at present comparatively shallow seas more abound with animal 
life than the extreme depths of the ocean. In these higher beds we 
first find traces of the diffusion of mechanical detritus in the pre- 
- sence of partings of shale, which begin to separate the beds of lime- 
stone ; and this increases until there is a pretty equal and regular 
alternation of beds of limestone and shale. Hereabouts, too, it is not 
unfrequent to find beds of fragments of shells, as if drifted along by 
currents. But here again we see proof of the lapse of time, since 
_ there must have been alternate periods, in one of which currents 
swept down fine mud into the sea, and deposited it in thin layers, 
and in the other a time of tranquillity, in which limestone was pre- 
cipitated. These alternations, too, are very numerous, since beds 
about 100 feet in thickness are thus constituted, each layer rarely ex- 
ceeding Ift. Ginches. Gradually, however, the mud-bearing currents 
increased in frequency, and the limestone becomes of more rare oc- 
currence, till at length it entirely ceases, and with it almost all trace 
of animal existence. Still the deposition must have been slow, since 
the lamine of the shale are of paper thinness. After two or three 
hundred feet of shale had been deposited, the currents increased also 
in strength, and became capable of sweeping in fine sand, which 
formed the beds of gritstone that begin to alternate with the shale 
in its upper portion. Another hundred feet of alternations of shale 
and gritstone now succeeds, in which the beds of grit gradually in- 
crease as we ascend, until at length we find great thick masses of 
gritstone, with merely thin beds of shale between them. In these 
thick masses of gritstone we see every mark of a much more rapid 
accumulation ; coarse grains, and even small pebbles, have been 
swept along, false bedding, or the heaping of layers of sand in slop- 
ing positions, continually occurs, and everything denotes the pre- 
sence of rapid currents sweeping in quantities of mechanical detritus 
from no very distant shores. This idea, moreover, is strengthened 
by finding the impressions or the trunks of broken plants, in a rude 
