~ . . z ~ 
Parr II. Szcr. iii. (1) § 2.] OLD RED SANDSTONE. 715 
Founding upon the absence from these northern rocks of the characteristic 
_ cephalaspidean fishes of the admitted Lower Old Red Sandstone of the 
south of Scotland and of Wales and Shropshire, upon the presence of 
numerous genera of fishes not known to occur in the true Lower Old 
Red Sandstone, and upon the discovery of a Pterygotus in the basement 
red sandy group of strata, he concluded that the massive flagstone series 
of Caithness could not be classed with the Lower Old Red Sandstone, 
but must be of younger date. He supposed these red sandstones, con- 
glomerates, and shales at the base, with their Pterygotus, to represent 
the true Lower Old Red Sandstone, while the great flagstone series 
with its distinctive fishes was made into a middle division answering in 
some of its ichthyolitic contents to the Middle Devonian rocks of the 
Continent. This view has been accepted by geologists. Recently, 
however, I have endeavoured to show that the Caithness flagstones 
belong to the Lower Old Red Sandstone, and that there is no evidence 
of the existence of any middle division. It appears to me that the 
discrepance in organic contents between the Caithness and the Ar- 
broath flags is by no means so strong as Murchison supposed, but 
that several species are common to both. In particular, I find that 
_ the characteristically Lower Old Red Sandstone and Upper Silurian 
crustacean genus Pterygotus occurs, not merely in the basement zone of 
the Caithness flags, but also high up in the series. The genera Acan- 
_thodes and Diplacanthus are abundant both in Caithness and in Forfar- 
shire. Parexus incurvus occurs in the northern as well as the southern 
basin. The admitted paleontological distinctions are probably not 
greater than the striking lithological differences between the strata df 
the two regions would account for, or than the contrast between the 
ichthyic faunas of contiguous water-basins at the present time. 
_ Somewhere about sixty species of fishes have been obtained from the 
Old Red Sandstone of the north of Scotland. Among these the genera 
Acanthodes, Asterolepis, Cheiracanthus, Cheirolepis, Coccosteus, Diplacanthus, 
Diplopterus, Dipterus, Glyptolepis, Osteolepis, and Pterichthys are specially 
characteristic. Some of the shales are crowded with the little ostracod 
crustacean Hstheria membranacea. Land plants abound, especially in the 
higher groups of the flagstones, where forms of Psilophyton, Lepidodendron, 
Stigmaria, Sigillaria, Calamites, and Cyclopteris, as well as other genera, 
occur. In the Shetland Islands traces of abundant contemporaneous 
volcanic rocks have been observed. These, with the exception of two 
trifling examples in the region of the Moray Firth, are the only known 
instances of volcanic action in the Lower Old Red Sandstone of Lake 
Orcadie. In the other two Scottish basins, those of the Cheviot Hills 
and of Lorne, volcanic action continued long vigorous, and produced 
thick piles of lava, like those of Lake Caledonia. 
Upper.—Below the Carboniferous system there occur in Scotland 
certain red sandstones, deep red clays or marls, conglomerates, and 
breccias, the sandstones passing into yellow or even white. ‘hese strata, 
wherever their stratigraphical relations can be distinctly traced, lie un- 
conformably upon every formation older than themselves, including the 
Lower Old Red Sandstone, while on the other hand they pass up 
conformably into the Carboniferous rocks above. Studied from the side 
of the underlying formations, they seem naturally to form part of the 
Old Red Sandstone, since they agree with it in general lithological 
