GEOLOGY OF LESMAHAGO. A7 
the Crustacean fossils, we afterwards endeavoured to obtain a general notion of the 
relations of all the rock-masses of the district.” * * * * * 
“ General Relations of the Rocks 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, 7. 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 Bonnington 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 
} ‘Quart. Journ. Geol. Soc.,’ 1851, vol. vii, p. 137. 
