496 Proceedings of the Royal Society 
the south-west of Ayrshire. Three distinct divisions have been 
ascertained — an upper series, graduating npwards into the Carboni- 
ferous formation, well seen in East Lothian ; a middle group, exten- 
sively developed, to the south of the town of Ayr ; and a lower 
group, which reaches an enormous thickness, and passes down into 
the Upper Silurian. In this lower division, as it approaches its 
north-eastern limit, a remarkable local unconformity has been 
ascertained by the recent researches of Mr B. N. Peach and myself. 
In the Pentland Hills the conglomerates and sandstones rest upon 
the vertical abraded edges of the Upper Silurian rocks ; while only 
a few miles distant, in the parish of Lesmahagow, there is no such 
break, hut the one series passes regularly into the other. The 
Pentland Hills, therefore, contain two groups of strata, both belong- 
ing to the lower Old Red Sandstone, but the one lying quite 
unconformably on the other. It seems not improbable that the 
great development of contemporaneous igneous rocks associated 
with these strata in the Pentland area may have had something to 
do with the origin of this local break in the succession of the 
deposits.* Much attention has been bestowed by the Survey dur- 
ing the last few years upon the lower members of the Carboniferous 
system. The result is that a twofold division has been made of that 
hitherto vaguely defined set of strata called by the late Mr Charles 
Maclaren the Calciferous Sandstones, and lying between the top of 
the Old Red Sandstone and the base of the Carboniferous limestone. 
New light has consequently been thrown upon the history of the 
earlier portions of the Carboniferous period in Scotland. The lower 
part of the Calciferous sandstones consists of a thick but variable 
group of red sandstones extensively developed in Ayrshire, Arran, 
* In Vol. V. of the Proceedings of this Society, p. 360, 1 have given a sec- 
tion of the Pentland Hills, which remains true, though I have since learnt more 
of the relations of the rocks there shown to the structure of this country at 
large. The series in the section marked c is stated to be “ a middle division 
of the Old Red Sandstone,” which is locally true, though more extended 
researches show that the great discordance between c and b, disappears in the 
course of a few miles, c is in reality only an upper unconform able portion 
of b. Again, the relation of the strata marked e to those below is cor- 
rectly shown in the section, but I am now convinced that the red sandstones 
of the Cairn Hills (e) are only a prolongation of that great band of red sand- 
stones which forms the base of the Carboniferous series throughout the west 
of Scotland. 
