440 
ELEMENTS OF GEOLOGY. 
sence of those marine forms by which we are able to identify 
or contrast marine formations, may be explained, while the 
great thickness of the rocks, which might seem at first sight 
to require a corresponding depth of water, can often be 
shown to have been due to the gradual sinking down of the 
bottom of the estuary or sea where the sediment was accu¬ 
mulated. 
Another active cause of local variation in Scotland was the 
frequency of contemporaneous volcanic eruptions; some of 
the rocks derived from this source, as between the Gram¬ 
pians and the Tay, having formed islands in the sea, and 
having been converted into shingle and conglomerate, be¬ 
fore the upper portions of the red shales and sandstones were 
superimposed. 
The dearth of calcareous matter over wide areas is charac¬ 
teristic of the Old Red Sandstone. This is, no doubt, in 
great part due to the absence of shells and corals; but why 
should these be so generally wanting in all sedimentary 
rocks the color of which is determined by the red oxide of 
iron ? Some geologists are of opinion that the waters im¬ 
pregnated with this oxide were prejudicial to living beings, 
others that strata permeated with this oxide would not pre¬ 
serve such fossil remains. 
In regard to the two types, the Old Red Sandstone and 
the Devonian, I shall first treat of them separately, and then 
allude to the proofs of their having been to a great extent 
contemporaneous. That they constitute a series of rocks in¬ 
termediate in date between the lowest Carboniferous and 
the uppermost Silurian is not disputed by the ablest geolo¬ 
gists; and it can no longer be contended that the Upper, 
Middle, and Lower Old Red Sandstone preceded in date the 
three divisions to which, by aid of the marine shells, the De¬ 
vonian rocks have been referred, while, on the other hand, we 
have not yet data for enabling us to affirm to what extent 
the subdivisions of the one series may be the equivalents in 
time of those of the other. 
Upper Old Bed Sandstone. —The highest beds of the series 
in Scotland, lying immediately below the coal in Fife, are 
composed of yellow sandstone well seen at Dura Den, near 
Coupar, in Fife, where, although the strata contain no mol- 
lusca, fish have been found abundantly, and have been re¬ 
ferred to the genera Iloloptychius^ Pamphractus^ Glyptopo- 
mus^ and many others. In the county of Cork, in Ireland, a 
similar yellow sandstone occurs containing fish of genera 
characteristic of the Scotch Old Red Sandstone, as for ex¬ 
ample Coccosteus (a form represented by many species in the 
