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OLD RED SANDSTONE OF WESTERN EUROPE. 365 
different rivers. The still more important barrier of the Rocky Mountains 
separates ichthyological areas yet more sharply marked off from each other. 
Such isolated basins as Lake Baikal, Lake Titicaca, and the Caspian Sea 
show by their peculiar assemblages of fishes how much ichthyic types may be 
modified by prolonged isolation. The differences, therefore, between the fauna 
of Lake Orcadie and Lake Caledonia during the Old Red Sandstone, as I ven- 
ture to hold, are not incompatible with the idea that the two lakes were in a 
general and geological sense contemporaneous, though separated from each 
other by the barrier of the Grampian Mountains, which formed an effectual 
boundary between two ichthyic faunas. 
In the third place, it can be shown that even among the tracts covered by 
undoubted Lower Old Red Sandstone considerable divergences in the fishes and 
crustaceans exist, pointing to separate areas of deposition. It is doubtful if a 
single species of fish or crustacean found in the Lower Old Red Sandstone of 
England and Wales can be identified with one found in Scotland. Most of 
the genera, however, are the same. This certainly indicates the influence of 
geographical distribution, though it shows that the barrier between the English 
and Scottish basins was not so complete, or had not existed for so long time, 
as that which interposed between Lake Caledonia and Lake Orcadie. 
5. So far, therefore, as the decision of the question rests upon evidence 
obtainable within the area of the British Islands, I submit that the presumption 
is in favour of the Old Red Sandstone on the north side of the Scottish High- 
lands being the same as that on the south side, and that consequently the 
so-called middle division of the system does not really exist there. So strong 
does this presumption seem to me that it cannot, I think, be set aside, save by 
much more convincing reasoning than that by which the Middle Old Red Sand- 
stone of this country was considered to be established. 
But Sir R. Murcuison was probably quite aware of the weakness of his 
argument, so far as the case of the British rocks are concerned. He endeavoured 
to strengthen it by an appeal to the Old Red Sandstone of Russia, where, he 
asserted, the fishes of his Middle Old Red Sandstone group of Caithness and 
the Moray Firth occur in the very same beds with true Middle Devonian shells. 
He laid great stress on this argument, regarding it deed as unanswerable in 
itself and as affording the most cogent possible proof of the soundness of his 
threefold classification of the Devonian or Old Red Sandstone system. And 
certainly it appears at first to deserve the confidence which he reposed in it. 
But an inquiry into the facts on which he depended, shows that to some extent 
at least he was misled. 
In the first place, in making the comparison between the Scottish and 
Russian ichthyolites Murcuison availed himself of fossil lists wherein the two 
very distinct Old Red Sandstone groups of the Moray Firth were not suffi- 
VOL. XXVIII. PART II. 5 C 
