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712 STRATIGRAPHICAL GEOLOGY. — [Boox VI. 
not infrequently become fossiliferous. At the same time it is not un-. 
‘worthy of remark that some of the red conglomerates, which might be 
supposed little likely to contain organic remains, are occasionally found | 
to be full of detached scales, plates, and bones of fishes. 
The Old Red Sandstone of Britain, according to the author’s re- 
searches, consists of the following subdivisions: 
2. Upper.—Yellow and red sandstones, conglomerates, marls, &c., 
passing up conformably into the base of the Carboniferous system, and 
resting unconformably on the Lower Old Red Sandstone and every older 
formation—Holoptychius, Pterichthys major, &c. 
1. Lower.—Red sandstones, conglomerates, flagstones, and associated 
igneous rocks, passing in some places conformably down into Upper 
Silurian formations—Diépterus, Coccosteus, Cephalaspis, Pterygotus, &c. 
Lower.—In a memoir on the Old Red Sandstone of Western Hurope, 
the author has proposed short names for the different detached basins 
in which the Lower Old Red Sandstone was accumulated.! The most 
southerly of these (the Welsh Lake) lies in the Silurian region extending 
from Shropshire into South Wales. Here the uppermost parts of the 
Silurian system graduate into red strata, not less than 10,000 feet thick, 
which in turn pass up conformably into the base of the Carboniferous 
system. This vast accumulation of red rocks consists in its lower 
portions of red and green shales and flagstones, with some white sand- 
stones and thin cornstones; in the central and chief division, of red and 
green spotted sandy marls and clays, with red sandstones and corn- 
stones; in the higher parts, of grey, red, chocolate-coloured, and yellow 
sandstones, with bands of conglomerate. No unconformability has yet 
been detected in any part of this series of rocks, though, from the obser- 
vations of De la Beche, it may be suspected that the higher strata, which 
graduate upward into the Carboniferous formations, are separated from 
the underlying .portions of the Old Red Sandstone by a distinct 
discordance. 
Although, as a whole, barren of organic remains, these red rocks 
have here and there, more particularly in the calcareous zones, yielded 
fragments of fishes and crustaceans. In their lower and central portions 
remains of the ganoids Cephalaspis, Didymaspis, Scaphaspis, Pteraspis, and 
Cyathaspis have been found, together with crustaceans of the genera 
Stylonurus, Pterygotus, and Prearcturus, and obscure traces of plants. The 
upper yellow and red sandstones contain none of the cephalaspid fishes, 
which are there replaced by Pterichthys and Holoptychius, associated with 
distinct impressions of land-plants. In some of the higher parts of the 
Old Red Sandstone of South Wales and Shropshire, Serpula and Conularia 
_ occur; but these are exceptional cases, and point to the advent of the 
Carboniferous marine fauna, which doubtless existed outside the British 
area before it spread over the site of the Old Red Sandstone basins. 
It is in Scotland that the Old Red Sandstone shows the most com- 
plete and varied development, alike in physical structure and in organic 
contents. Throughout that country the system is found everywhere to 
present a division into two well-marked groups of strata, separated from 
each other by a strong unconformability and a complete break in the suc- 
cession of organic remains. It occurs in distinct basins of deposit. One 
of these occupies the central valley between the base of the Highland 
1 Trans, Roy. Soc. Edin, vol. xxviii. 1879. 
