Chap. VIII.] SILURIAN EOCKS OF SCOTLAND. 
153 
by the Wrae limestone of Peebles-shire, b 3 , which, from its organic remains 
(Trilobites and Shells), has been paralleled with the Llandeilo limestone*. 
All the associated schists and other strata were therefore termed Silu- 
rian by Professor Nicol (1846), the more so as the fossiliferous limestone 
reposed on schists containing Graptolites. In pursuing this traverse north- 
wards, other overlying schists and slaty rocks are seen to overlie the Llan- 
deilo (?) limestone of Wrae, though these are soon lost under and covered 
over by the Old Eed Sandstone, /, and the Carboniferous formation of the 
Lothians, g. Beneath those deposits, /, g, we know not what flexures may 
occur in the subjacent or older rocks of the great intervening trough ; but 
on reaching the Pentland Hills, hard schistose strata are again met with, 
nearly parallel to those we have left, and, like them, inclined to the NT.N".W. 
at high angles. Under these circumstances only, it would be difficult to 
assign to the constituent strata of the Pentland Hills any more definite 
place in the series than that of Upper Silurian. Previously to the year 
1839, the late Mr. Charles Maclaren had found an Orthoceras in these 
strata, which most resembled a "Wenlock species ; in 1858 Mr. Geikie 
detected Rhynchonella compressa, also a Wenlock species ; and from this 
evidence it was inferred in the last edition of this work that these Silurian 
rocks of the Pentland Hills were of "Wenlock age. Since that time Mr. 
Geikie has shown that they contain representatives of the Ludlow rocks, 
as will be pointed out a little further on. 
A second traverse of the chain, near the eastern coast, or towards the 
sea-board of the Lammermuir Hills, exposes the lower members of the 
series just described. There the Teviotdale anticlinal axis, a, is marked 
by the course of the Lower Tweed. The purple schist seen to the north of 
Berwick, the outlines of which have been sketched (p. 150), appears, there- 
fore, to belong to the same bottom rocks of the series. Further north- 
wards, higher beds, the equivalents of the Peebles-shire groups, occur ; and 
in these, on the Dye near Byreclugh, Mr. Stevenson of Dunse f has found 
a Graptolite, an obscure Coral, and slates with Annelide-impressions. As 
these beds lie in the line of strike of the Grieston and Thornielee slates, 
they probably coincide with them in age. The predominating dip of the 
strata is to the north-west, showing a general ascending section, notwith- 
standing the numerous convolutions, which are beautifully seen on the 
coast, and have been already adverted to. This region has recently been 
mapped and discribed in detail by Mr. Geikie, during the progress of the 
Geological Survey in Scotland J. He notices the occurrence of Graptolithus 
priodon at Siccar Point, of Diplograpsus pristis in dark shales at the head 
* Among the fossils of the Wrae limestone Foss. Mus. Cambridge,' pp. 248, 374, &e. 
found by Professor Nicol are : — Illaenus Bow- t See Proc. Geol. Soc. Lond. vol. iv. pp. 29 and 
manni, Salter ; Harpes parvulus, M'Coy ; Asa- 79 ; and Quart. Journ. Geol. Soc. Lond. vol. vi. 
?hus tyrannus, Sil. Syst. ? ; a species of Phacops ; p. 418. I long ago examined the purple greywacke 
iheirurus ; Orthoceras areuoliratum, Hall ; Or- or bottom rock, near Dunse, in company with Mr. 
this calligramma, Dalman; O. biforata, Schlot- William Stevenson. 
heim ; Strophomena tenuistriata, Sil. Syst. ; J See Geology of Eastern Berwickshire (Mem. 
Salter, in Quart. Journ. Geol. Soc. Lond. vol. iv. Geol. Surv. 1864), chap, ii., and Geology of East 
p. 205, and Professor M'Coy in the ' Brit. Pal. Lothian (ib. 1866), chap. ii. 
