OLD RED SANDSTONE OF WESTERN EUROPE. 363 
upper limit of the Caithness series, I shall prove from clear natural sections 
that it is marked by a strong unconformability, with which the overlying Upper 
yellow and red sandstones make their appearance. In short, there are in the 
north of Scotland only two great divisions of the Old Red Sandstone, and it 
seems at least probable that they represent the two divisions which occur 
everywhere else in Britain. 
3. There can be no doubt that lithologically the Old Red Sandstone of the 
north of Scotland differs in a marked way from that of any other district within 
these islands. So striking, indeed, is this divergence, that even a geologist 
whose eye has long been familiar with the Old Red Sandstone in more southern 
regions, finds at first some difficulty in believing that the dark shaly flagstones 
and limestones of Caithness and the Orkney Islands can form part of the Old 
Red Sandstone within the same geographical area in which red and purple 
sandstones and conglomerates are elsewhere so prevalent. In other tracts he 
is familiar with the general barrenness of the red sandy strata in regard to 
organic remains ; but in these northern rocks he meets with layers which are 
crowded with well-preserved teeth, scales, and bones of fishes. Mere difference 
of lithological character does not necessarily point to difference of age, and 
certainly cannot be cited as evidence in favour of a threefold classification of 
the Old Red Sandstone. But this striking contrast in the nature of the strata 
does point to markedly dissimilar conditions of deposit. The Old Red Sand- 
stone of the north of Scotland must have accumulated in a distinct geographical 
basin, and under circumstances to which as yet no parallel has been found in 
the Old Red Sandstone of other parts of Britain. So singularly characteristic 
is this lithological discrepancy, that the observer who comes upon it for the 
first time naturally expects to find it accompanied by a corresponding paleon- 
tological divergence. He feels that the Caithness and Orkney flagstones 
are themselves so peculiar that the waters in which they were laid down 
might reasonably be expected to have been tenanted by other forms of life 
than those which have been preserved among the sandstones and shales of 
Forfarshire. 
4. It was on the fossil evidence that Sir R. Murcutison chiefly relied when 
he proposed his “ middle” Old Red Sandstone group. His position seems to 
be strong and well chosen. Among the numerous ichthyolites of the Caithness 
flagstones, none of the cephalaspids so characteristic of the Lower Old Red 
Sandstone of Forfarshire and Herefordshire occur. The Caithness fishes, on 
the other hand, as he contended, are not represented in the Lower Old Red 
Sandstone. Deposits differing so much paleontologically cannot therefore be 
contemporaneous. The Caithness flagstones must thus be younger than the 
cephalaspid sandstones which are found passing down into the Upper Silurian 
Series. On the other hand, they are certainly older than the Holoptychius- 
