awit 
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716 |  STRATIGRAPHICAL GEOLOGY. [Boo VI. 
character, and also in containing some distinctively Old Red Sandstone — 
genera of fishes, such as Pterichthys and Holoptychius ; though, approached — 
from the upper or Carboniferous direction, they might rather be assumed 
as the natural sandy base of that system into which they insensibly 
graduate. On the whole, they areremarkably barren of organic remains, 
though in one locality—Dura Den in Fife—they have yielded a number 
of genera and species of fishes, crowded profusely through the pale 
sandstone as if the individuals had been suddenly killed and rapidly 
covered over with sediment. Among the characteristic organisms of 
the Scottish Upper Old Red Sandstone are Pterichthys major, Holoptychius 
nobilissimus, H. Andersoni, Glyptopomus, Glyptolemus, and Phaneropleuron. 
An interesting fact deserves mention here as a corollary to what has 
been stated above regarding the survival for some time of an Upper 
Silurian fauna outside the area of the British Old Red Sandstone lakes. — 
In the Upper Old Red Sandstone of the basin of the Firth of Clyde, 
Ptericthys major and Holoptychius occur at the Heads of Ayr, while a 
band of marine hmestone lying in the heart of the red sandstone series 
in Arran is crowded with ordinary Carboniferous Limestone shells, such 
as Productus giganteus, P. semireticulatus, P. punctatus, Chonetes hardrensis, 
Spirifera lineata, &c. None of these fossils has been detected in the great 
series of red sandstones overlying the limestone. They do not reappear 
till we reach the limestones in the Lower Carboniferous series ; yet the 
organisms must have been living during all that long interval outside of 
the Upper Old Red Sandstone area (p. 739). Not only so, but they must 
have been in existence long before the formation of the thick Arran — 
limestone, though it was only during the comparatively brief interval 
represented by that limestone that geographical changes permitted them 
to enter the Old Red Sandstone basin and settle for a while on its floor. 
Thus we see that while, on the one hand, the older parts of the Lower 
Old Red Sandstone were coeval with an Upper Silurian fauna which, 
having disappeared from the area of Britain, survived outside of that - 
area, on the other hand, the higher parts of the Upper Old Red Sandstone 
were contemporaneous with a Carboniferous Limestone fauna which, 
having appeared beyond the British area, was ready to spread over it as 
soon as the conditions became favourable for the invasion. It is, 
of course, obvious that such an abundant and varied fauna as that of 
the Carboniferous Limestone cannot have come suddenly into existence 
at the period marked by the base of the limestone. It must have had 
a long previous existence outside the present area of the deposit. But 
it is seldom that we obtain such clear evidence of this paleontological 
relation as in these instances from the Scottish Old Red Sandstone. 
In the north of Scotland, on the lowlands bordering the Moray Firth, 
and again in the island of Hoy, one of the Orkney group, yellow and red 
sandstones, sometimes containing characteristic Upper Old Red Sand- 
stone fishes, are found lying unconformably upon the Caithness flags. 
In these northern tracts the same relation is thus traceable as in the 
central counties between the two divisions of the system. 
Turning southward across the border districts, we trace the red 
sandstones and conglomerates of the Upper Old Red Sandstone lying 
unconformably on Silurian rocks and Lower Old Red Sandstone. Some 
of the brecciated conglomerates have much resemblance to glacial 
detritus, and it has been suggested that they have been connected with 
