266 
VOLCANOES OF THE LOWER OLD RED SANDSTONE 
HOOK V 
s]jeci(!s are often distinct. Two large lake.s, separated by tlie tract of tlie 
Scottisli Highlands, had each its own assemblage of fishes, not a single 
genus being common to the two basins. .Such contrasts, whether the two 
lakes were geologically contemporaneous, or the northern ai’ose later than 
the soutliern, undoubtedly indicate long-continued isolation and the gradual 
evolution of new forms under different conditions of environment.^ 
Such, in brief, were the aspects of the physical geography of the time on 
the further consideration of which we are now to enter. Tlie subterranean 
disturbances, so characteristic of the period, were accompanied liy a displav 
of volcanic activity more widespread, perhaps, than any which had yet taken 
place in the geological history of Britain. Nevertheless, it is worthy of remark 
that this manifestation of underground energy did not begin with the com- 
mencement of these displacements of the crust. The earliest eruptions only 
took ])lace after the geography of the I’egion had been completely changed ; 
at least no trace of them is to be found in the earliest portions of the Old 
Red Sandstone. After the last lingering Silurian volcanoes in the west of 
Ireland liad died out, a jirotracted (piicscence of the subterranean fires 
ensued. In the latest ages of .Silurian time there was not in Britain, so far 
as at present known, a single volcanic eruption. Not until after the in- 
auguration of the Old Red_ Sandstone topography, when the lakes had 
taken shape and had begun to be filled witli sediment from the surrounding 
hills, did a series of new volcanoes burst into activity over the northern 
half of Britain. Rising in the midst of the lakes in groups of .separate 
cones, these vents poured out floods of lava, together with clouds of ashes 
and stones. Their sites, tlie history of their eruptions, and the piles of 
material ejected by them, can still be ascertained, and T shall now proceed 
to give some account of them. 
The thick mass of sedimentary material known as the Old Red Sand- 
stone, lying between the top of the Silurian and the base of the Carbon- 
iferous system, has been divided into two sections, which, however, are of 
unequal dimensions, and doubtless represent very unequal periods of time. 
The older .series, or Lower Old Red .Sandstone, is by far the more important 
and interesting in its extent, thickness, palnsoutological riches, and, what 
' In iiiy memoir “On the Old Red .Sandstone of Westei'n Europe” {Trans. Hoy. Soe. Min. 
vol. xxviii. 1878), I argued lor the probable geological contemporaneity of the conglomerates, 
sandstones and llag,stone.s on either side ol the Gi’ampian chain, even although their organic 
contents were so unlike. The stratigr.a]ihical evidence favours this view. In each case a thick 
series of strata is covered unconformably by Uiiiier Old Rod .Sandstone, conUining Hoh-pUjehius 
nobilissimus and other (i.shes. The question cannot perhajis be definitely settled by the data 
available in .Scotland. It is quite 2)0.ssible that the basin on the northern side of the tfranipians, 
which I have termed “Lake Orcadie,” came into existence after that on the southern side. But 
I do not think the difterenecs in their resjiective faunas are to be accounted for sinq.ly by laj.sp 
of time and the gradual organic evolution in j.rogress over one continuous region. The more 
the Old Red Sandstone is studied, the more local do its various tish-liiunas ajqiear to have been. 
These strongly -marked diversities appear to me rathei' to point to j.rolonged isolation of the 
basins from each other, as st.ated above. Hr. Tra.juair has drawn .attention to the remarkable 
fact that, even in what ajqiears to be one eontinuoits series of .str.ata of no great thickness form- 
ing the Upjrer Old Red Sandstone ol the Moray Eirth basin, the fishes fonnd .about Xuirn are 
entirely dillerent from those met with i!i the rest of the region. 
