﻿GEOLOGY OF LESMAHAGO. 47 



the Crustacean fossils, we afterwards endeavoured to obtain a general notion of the 

 relations of all the rock-masses of the district.'' ***** 



" General Belations of the BocJcs of the Lesmahago District. — In a former com- 

 munication I invited attention to the general direction of the great masses of the 

 Silurian rocks of the South of Scotland, which have been described by various authors 

 under that name, since the discovery in them of many well-known Silurian fossils. 1 I 

 then suggested that, judging from some of those organic remains, as found in the 

 environs of Girvan, there were indications, in that parallel, of an ascending order 

 from the Lower Silurian rocks (which unquestionably form the great mass of the South- 

 Scottish Greywacke) to the Upper Silurians. At the same time it was noticed that 

 the strike of the Girvan strata would carry them nearly to the Silurian rocks of the 

 Pentland Hills, south of Edinburgh, which have the same general direction, i. e. nearly 

 from north-east to south-west. Now, if a line be drawn from the rocks north of Girvan 

 to the northern face of the Pentland Hills, it is seen to pass over an intervening tract, 

 throughout which basins of Carboniferous rocks, surrounded by girdles of Old Red 

 Sandstone and diversified by a great abundance of igneous rocks (porphyry, greenstone, 

 &c), are represented in all the published geological maps. The discovery made by Mr. 

 Slimon of fossils which prove to be of Upper Silurian age, over a considerable area in the 

 extensive parish of Lesmahago (for this Scottish parish has a length of twenty-five miles), 

 has advanced, therefore, the northern frontier of the Silurian or slaty rocks ; some of the 

 localities in question being not less than twenty miles to the north-west of their previously 

 defined boundary. The extent to which the Lesmahago Silurians may be hereafter shown 

 to be connected upon the surface with those of the tracts around the Lead Hills, and 

 other parts of Ayrshire and Dumfriesshire, on the south and south-east, must be a work 

 of future labour. That work will demonstrate whether these Lesmahago rocks constitute 

 an advanced Silurian promontory or headland, or whether, as is most probable, they form 

 an outlier of that age. 



" The large parish of Lesmahago is pre-eminently distinguished in its western part 

 by dome-shaped hills, which rise to the south-west of the River Clyde, and on the left bank 

 of that river, where it forms the well-known beautiful Falls of Ronnington and Stone- 

 byres, near the town of Lanark. The rock over which the Clyde cascades is the Old 

 Red Sandstone, which formation, extending to the west and south-west to the village and 

 parish of Lesmahago, is overlain on the north and south by Carboniferous Limestones 

 and Coal, whilst on the south-west it is underlain by the dark and schistose rocks to 

 which attention is now specially invited. 



" Upper Silurian Rocks. — The relations of these dark grey, schistose strata of clay-slate 

 1 'Quart. Journ. Geol. Soc.,' 1851, vol. vii, p. 137. 



