OLD RED SANDSTONE OF WESTERN EUROPE. 361 
belong to the Lower Old Red Sandstone, but must be younger than the Forfar- 
shire flagstones, in which such old forms as Cephalaspis and Pterygotus occur.* 
Accordingly he grouped them as a “ middle” division, in conformity with the 
triple classification so much in vogue. He sought to establish an analogy 
between the three groups of the Old Red Sandstone and the three groups of 
the Devonian system, maintaining that they are essentially the equivalents one 
of the other. So far as I have been able to gather, the following reasons appear 
to have guided him to these conclusions. He believed and affirmed—1s¢, That 
while in the true Lower Old Red Sandstone several genera of fishes and crusta- 
ceans occur (Péeraspis, Pterygotus, &c.), which descend into the Upper 
Silurian rocks, not one of these occurs in the flagstone series of the north of 
Scotland. 2d, That the ichthyic fauna of the latter series, differing so greatly 
as it does from that of the true Lower Old Red Sandstone, cannot be of the 
same age, and from the absence of such early forms as Pteraspis, Cephalaspis, 
&c., must be presumed to be of younger date. 3d, That at the base of the 
flagstones of Caithness, red sandstones and conglomerates, in which Mr C. W. 
Pracu found Pterygotus, may be recognised as equivalents of the Lower Old 
Red Sandstone, occupying their proper place below the great “ middle ” for- 
mation with its abundant and peculiar fishes. 4th, That the upper portion 
of this middle formation passes upwards in the north of Scotland into the 
Upper Old Red Sandstone, which it could hardly have been expected to 
do had it really been of so ancient a date as the flagstones of Forfarshire. 
5th, That the fishes of the ‘“‘ middle” Old Red Sandstone of Scotland occur in 
the middle Devonian rocks of Russia, thus showing so remarkable a corre- 
spondence between the Old Red Sandstone and the Devonian groups in the 
west and east of Europe as to justify the conclusion that they are mutually 
equivalent. 
Now it is impossible to deny the ingenuity and apparent cogency of this 
reasoning. Mr Satter, in the passage already cited, declared it to be “a 
masterly suggestion,” and “ the greatest advance made of late years in the classi- 
fication of the British Devonian rocks.” + I venture to think, however, that 
the argument is based on such evidence as to render it more than inconclusive. 
Some of the supposed facts on which it rests may be shown to be erroneous ; 
while another, and, as it seems to me, quite as probable an interpretation, may 
be put upon those which remain unchallenged. After long research in the field, 
and much anxious consideration of the whole subject, I am unable to find any 
valid reason for the erection of the so-called ‘Caithness flagstones ” into a 
“middle” division of the Old Red Sandstone. Nowhere in the British area, so 
* “Quart. Journ. Geol. Soc.” xv. p. 400; and “ Siluria,” 3d edit 1859, p. 284. 
+ “Quart. Journ. Geol. Soc.” xix. 493. 
VOL. XXVIII. PART II. 0B 
