- 362 PROFESSOR GEIKIE ON THE 
‘far as I know, does any group of rocks exist which requires to be ranked as 
Middle Old Red Sandstone. I can discover only two great well-marked series 
of strata, which build up the Lower and Upper divisions of this Pale- 
ozoic system of deposits. The Old Red Sandstone of Lake Orcadie I 
would place in the Lower series, and would explain its peculiarities by 
the geographical circumstances under which it was laid down, This conclu- 
sion has been forced upon me by the evidence which I shall now proceed to 
adduce. 
1. Taking the British area, we have nowhere any strata which could 
possibly be claimed as forming a distinct “middle” series save those of the 
north of Scotland. Everywhere else the twofold classification into Lower and — 
Upper is manifest. Yet to the south of the Grampian range these two sub- 
divisions attain the vast development of more than 20,000 feet. In South 
Wales, and again in Ireland, they attain an enormous thickness. On the view 
which I am combating we must suppose that the break between these two 
-series is everywhere so great as to be chronologically equivalent to the whole 
of the vast depth of the “Caithness and Orkney flagstones.” Though the 
Lower and Upper Old Red Sandstones are certainly strongly unconformable in 
many places, yet in wide. tracts, where derangements of strata are otherwise 
plentiful, they lie with so little discordance that it is difficult to believe the 
interval between their deposition to have been so vast. 
Though arguments founded on the thickness of strata are notoriously un- 
safe, I may be allowed to point out that, on the supposition that the Old Red 
Sandstone of the north of Scotland forms a division of the system else- 
where unrepresented in Britain, the total thickness of the Old Red Sandstone 
would be increased to a united depth of between 30,000 and 40,000 feet. I 
prefer to reduce this thickness by making the Old Red Sandstones on the 
north side of the Scottish Highlands the general equivalents of those on the 
south. 
2. While in none of the tracts where Lower and Upper Old Red Sandstone 
‘occur is there any true ‘‘middle” group, in the north of Scotland, where the 
so-called ‘“‘middle” group attains so great a development, there is no repre- 
sentative of any lower series. Sir R. Murcuison affirmed, indeed, that the 
Caithness flagstones may be seen in various places to graduate downwards into 
‘equivalents of his Lower red sandstones, and upward into the Upper light red 
and yellow sandstones. I shall be able to show in this memoir that what he 
‘considered to be equivalents of the Arbroath flagstones or Lower Old Red 
Sandstone, form merely the variable and sandy or pebbly base of the Caithness | 
series, and occur on many different horizons throughout that series. They can- 
not be regarded as in any sense equivalents of the whole vast succession of 
sandstones and conglomerates in central Scotland. - With regard, also, to the 

