518 Proceedings of the Royal Physical Society. 
The stratigraphical relations of the subdivisions of the 
Old Red in the south of Scotland were admirably described 
by Mr (now Sir Archibald) Geikie in volume xvi. of the 
Quarterly Journal of the Geological Society of London; and 
again (and, if possible, better still), in an oft-quoted descrip- 
tion of the facts in “ Siluria,” 4th edition, 1867, pp. 248-250. 
The Orcadian Rocks, specially referred to in the present 
paper, were described in detail in the same author’s “ Old 
Red Sandstone of Western Europe” (Zrans. Roy. Soc. Edin., 
vol. xxxvii.). For much of what we know respecting the 
paleontology of these rocks, and especially regarding the 
bearing of the evidence yielded by fossils upon the order of 
succession of the various subdivisions of the Old Red, we are 
indebted to the labours of Dr Traquair, whose beautiful and 
life-like restorations of the fish of the Devonian Period have 
now made the study of these fossils a pleasure to geologists, 
instead of a task of almost hopeless difficulty, as it has been 
hitherto. 
For the purpose at present before us, we need only to 
summarise from the writings of these and other authors the 
main facts regarding the succession, and the chief petro- 
graphical characters of each group of the Scottish rocks of 
Devonian age. 
At the bottom of the series occurs the Lanarkian “ Old 
ted” already referred to, which consists largely of red sand- 
stones, and which graduate downward into, and form part 
of, rocks of Ludlow Age. These Lanarkian Rocks, as I 
hope to be able to show presently, are distinctly not of 
marine origin, but were formed in either lagoons or 
inland lakes, principally under arid—if not under desert— 
conditions. They are coloured bright red by ferric oxide, 
they contain no traces of fossil plants, and no bituminoid 
matter. These Lanarkian Rocks have long been known in 
the Pentland area to be covered by the Caledonian Old Red, 
a violent unconformity marking the stratigraphical relations 
between the two sets of rocks. The Caledonian Old Red in 
its lower part, and where not of volcanic origin, differs from 
the Lanarkian Rocks in being coloured largely by ferric 
hydrate. The rocks on this horizon, therefore, are brown 
