OF THE TRAP-ROCKS OF SCOTLAND. 643 
Silurian hills between Dalmellington and Straiton must be the source from which 
that conglomerate was derived; and consequently that, in Nithsdale as well as in 
Perth, the volcanic forces were at work during the Old Red Sandstone period. 
There are few areas of Scotland that promise to reward the observer with more 
interesting results than the high ground on the borders of the Silurian region, in 
the south of Lanarkshire and Ayrshire. 
Allusion has been made to the felstones of the Campsie Hills and part of the - 
Ochils, as belonging to the era of the Upper Old Red Sandstone. Another, and 
perhaps a still more characteristic example of volcanic rocks of this period, occurs 
in the chain of the Pentland Hills. 
Trap-Rocks of the Upper Old Red Sandstone. 
After the movements which resulted in the uptilting of the Lower Old Red 
Sandstone and Upper Silurian of central Scotland, the Upper Old Red Sandstone 
began to be thrown down upon the edges of the inclined and fractured strata. 
The base of this part of the series is well seen in the southern prolongation of the 
Pentland Hills, where it consists of a great thickness of conglomerate and sand- 
stones resting on highly inclined and vertical Ludlow shales. The higher part of 
the series is intercalated with sheets of interbedded felstones and ashes, forming 
the northern part of the chain. Since these igneous rocks may serve as the type 
of contemporaneous traps of Old Red Sandstone date, I shall briefly sketch their — 
general features.* . 
The Pentland Hills reach a height of about 1900 feet above the sea, and 
stretch south-west from Edinburgh, until they merge into the moorlands of 
Lanark and Peebles. They are bounded on either side by long parallel faults, 
which have brought down the carboniferous strata, sometimes vertically, and 
even reversed, against the older parts of the chain. The basement rocks of the 
hills are highly inclined grits, and slates, or shales, some of which, as I have 
already mentioned, contain a great abundance of fossils; The general aspect of 
these fossils is regarded by Mr Sauter as clearly placing them on the horizon of 
the Ludlows. Upon the tilted and denuded edges of these Silurian beds, there 
rests a great thickness of conglomerates and grits, which, though they have not 
yet yielded any organisms, are unquestionably parts of the Upper Old Red Sand- 
stone. Northward, this series becomes thinner, until it finally disappears. Its 
upper part is intercalated with, and covered by, beds of felstone, which, swelling 
out towards the north, where the grits and conglomerates have thinned off, rest 
directly on the irregular surface presented by the upturned edges of the Silurians. 
* See Mr Mactaren’s “ Sketch of the Geology of Fife and the Lothians,” p. 124, and “ The 
Geology of Edinburgh,” in the Memoirs of the Geological Survey, where I have described the hills 
in detail. Compare also the Survey Map of this district, sheet 32 (Scotland). 
+ See Memoir to accompanying sheet, 32 of Geol. Survey of Scotland. 
