250 
SILURIA. 
[Chap. XI. 
it presents no lithological resemblance. More recent observations in Ayr- I 
shire, however, lead me to suspect that this middle series is not merely an 
unconformable upper part of the Lower Old Eed Sandstone, but forms a 
distinct member of the formation. Between the town of Ayr and the Yale 
of Girvan, and stretching eastwards towards Nithsdale, there occurs a j 
thick mass of reddish and chocolate-coloured sandstone, with large inter- j 
calated sheets of felspathic trap and ashy beds. This series is comparable j 
with that of the Pentland Hills. It rests unconformably on the Silurian 
rocks already described, and on certain red sandstones and conglomerates I 
which may represent the Lower Old Eed series ; and as it continues to dip i 
steadily northward for ten or fifteen miles, its thickness must be great. It 
is covered unconformably by Lower Carboniferous rocks, there being no 
Upper Old Eed Sandstone in the district. Near the Heads of Ayr, in a 
hard reddish-grey micaceous sandstone, lying in the upper part of the 
series, abundant plates of Pterichthys major have been found. (See sec- 
tions on pp. 156 & 157.) 
" a. The Upper Old Eed Sandstone (e of the section) occurs extensively 
in the eastern division of the southern half of Scotland. It has long been 
known at Dura Den in Fife, where its yellow sandstones have yielded Ho- 
loptychius, Pamphractus, and other Pishes. An outlier of it lies uncon- 
formably on the Lower Old Eed Sandstone at Clashbinnie, in the lower 
part of the basin of the Tay ; and another occurs in a similar position near 
Arbroath. In Haddingtonshire and Berwickshire it consists of a lower 
thick mass of coarse brecciated conglomerate, lying unconformably on the 
Lower Silurian greywacke, and of an upper series of red sandstones and 
marls containing Cyclopteris, Bothriolepis, &c, and passing up into the 
base of the Carboniferous system. 
" It thus appears that, between the Grampians and the Cheviot Hills, 
what has been called 1 Old Eed Sandstone ' consists of three zones : — a lower, 
graduating down into Upper Silurian shales ; a middle, bounded both above 
and below by an unconformable junction ; and an upper, which shades up ' 
into the Carboniferous rocks." 
It is highly satisfactory that the triple division of the Old Eed Sandstone, 
which I established by my observations in the North of Scotland, is thus 
supported by a correct survey of the country south of the Grampians. In 
the sequel, proofs will be adduced of a like triple classification of this 
natural group where it assumes the Devonian or more slaty structure. 
Indications of the presence of the uppermost member of the Silurian 
rocks also exist in Porfarshire, and particularly on the flanks of the Sidlaw 
Hills, where the grey paving-stones of Arbroath repose upon clay-slate, 
and contain the Crustacean Pterygotus Anglicus, which appears not to be 
identical with the great Pterygotus of Shropshire* (see pp. 238, 239). 
The following general section over a part of that region is given by Sir C. 
* Quart. Journ. Geo]. Soc. vol. vii. p. 169. 
