448 PROFESSOR GEIKIE ON THE 
. 4, But perhaps the argument which will have most weight in support of the’ 
position for which I am contending is based upon the fact that the fossils of 
the area south of the Ord are unequivocally those of the upper flagstones of 
Caithness and Orkney. The following species, common to both areas and not 
known. or very rare in the lower flagstones, sufficiently establish this asser- 
tion :—Acanthodes pusillus, Asterolepis Asmusi, Diplacanthus longispinus, D. 
striatus, Glyptolepis leptopterus, Pterichthys Milleri. Besides these, each area 
contains what are said to be distinct species of Cheiracanthus and Cheirolepis, 
two genera not yet found among the lower flagstones. I have little doubt, 
when the fauna of Lake Orcadie comes to be revised, that a still greater 
community of species will be shown to exist. 
Tf, then, the statement be accepted that the Lower Old Red Sandstone 
south of the Ord represents the upper portion only of the great Caith- 
ness flagstone series, an interesting light is thrown upon part of the history 
of Lake Orcadie. We see that during the greater part of that history the 
southern margin of the lake did not extend beyond the Ord. All south of 
that granitic ridge was land. The mainland of Scotland, consequently, had at 
that time a considerably greater extension northward than it can boast of now. 
The northern slopes of the Highlands extended over the area of the Moray 
Firth. The depression which went on to the north and carried down the 
bottom of the lake did not begin seriously to affect the ground south of the 
Ord until late in the history of the flagstones. But at last the land bordering the 
lake on its southern margin began to go down. The waters crept southward, 
until in the end they filled the hollows and glens leading up into the Grampians; 
and as we have seen, perhaps even penetrated into the very heart of these high 
erounds. During this subsidence, as the waters encroached on the land, banks 
of gravel would be formed along the shore at different successive levels, especi- 
ally in recesses of the coast-line like that of Cawdor. Some of the same fishes 
which lived in the more open water towards the north would find their way 
southward with the gradual encroachment of the lake. But in so doing, they 
passed into tracts where the conditions of the water and of the bottom were 
somewhat different, and where at intervals, now marked by the nodule-bearing 
clays, their bodies, when they sank upon the silt, served as centres round 
which calcareous matter was segregated. Such intervals were, however, com- 
_paratively infrequent during the whole of the period represented by the deposits 
of. the Moray Firth. As a rule, sand and gravel were the materials which 
there gathered on the lake bottom, and in these the traces of the fauna of the 
time are scarcely ever to be found. 

