The Clyde rises from the northern border of the great Silurian rocks of the Southern Highlands, flowing along these until about Symington, where it enters the Old Red and afterwards the Carboniferous basin onwards to Glasgow. The Silurian system of the south of Scotland is described as follows by Professor Young: “The broad undulating district lying to the south of the Carboniferous basin of Central Scotland, and known under the general name of the Southern Uplands, is carved almost wholly out of rocks of Silurian age. The dominant formation is an immense series of comparatively barren graywackes and shales, which, thrown into innumerable folds and contortions, spread in an unbroken

sheet from St. Abb’s Head to the Mull of Galloway, forming by far the grandest exhibition of Middle Silurian Strata yet discovered.” Page, in his Text Booh of Geology, says: “This system is largely developed in various countries, both in the Old and in the New World, and typically so in the district between England and Wales, anciently inhabited by the Silures; hence the designation ‘Silurian System’ by Sir It. Murchison, their first and most ardent investigator.”

The Clyde valley presents varied geological features, and offers a very good field for the study of this now important economical science. The rocks throughout are of the earlier and Carboniferous periods, seamed by dykes and capped by overflows of trappean rocks, indicating great changes of level and conditions in the past; whilst to the student of the glacial period abundant evidences of the presence of a great ice-sheet once more or less filling up the valleys, as the glaciers of Switzerland and Norway now do, may be met with in the boulders scattered about the lower levels, the great masses of boulder-clay, and the smoothed and striated rock surfaces which may still be met with. In reference to this it may be noted here that “A table-case in the Hunterian Museum contains a series of hand specimens, obtained by Mr. Young from the boulders which were removed when the summit of Gilmorehill was lowered for the foundation of the University. The series includes all varieties of rock from Bonawe to the Kilpatricks. The glacial striations of the district are generally from north-west to south-east.”

If we take a sectional view of the country, in a line running north and south through the Clyde valley and passing through the Glasgow district, we find the Silurian rocks appearing to north and south, as a framework on which rests deposits of Old Red Sandstone, on which, in turn, rest the limestone, shales, and coal and iron beds of the Carboniferous period. Ejections of trappean rocks are frequently met with in the later deposits, troubling the miner by causing upthrows and downthrows.

The most widely-spread and most interesting series, in an industrial point of view, is the Carboniferous, covering-all the middle portion of the Clyde valley, and extending to a depth of several thousand feet. The series consists mainly of beds of coal, iron-stone, shales, and fire-clay, with their accompanying limestones and sandstones.

The great “fault” already referred to as crossed by the Rotten Calder has caused the downthrow of the great coal-beds to a level with the lower deposits of the carboniferous limestone, shown in a geological map of the district by a sharp dividing line between the dark-coloured portion (the coal) and the bluish (the limestone). The amount of this displacement equals that of the thickness of the beds awanting, and has been estimated at about 1500 feet. Speaking of the limestones to the south of Glasgow Mr. Bell (Rocks around Glasgow) says:—

“They are also often called the ‘cement limestones,’ being largely used for cement and building purposes, as from a certain admixture of silica and alumina in their composition they have the property of ‘setting’ with a firm band under water. The Orchard limestone is wrought at a short distance to the south of Giffnock Quarries. It is also wrought as the ‘Lyoncross’ limestone at Nitshill and Barrhead, and is known as the ‘Williamwood’ limestone near Cathcart. It is only a thin bed, from 18 to 26 inches in thickness, but is of excellent quality, and has long been esteemed as a cement limestone. Underneath it is a thin seam of coal, which is used in calcining the stone. The Arden limestone, wrought extensively near Thornliebank and Barrhead, is in much greater mass, attaining a thickness of 8 to 10 feet, in some places even more. Its equivalent to the north of the Clyde, in the Garnkirk district, is largely used for iron-smelting.”

The varieties of coal, and varying condition and thickness of the limestone and sandstone beds, all point to long periods of land surface, with correspondingly extended periods of depression under the surface of the water.

Professor Geikie, in his Scenery of Scotland, thus graphically describes the Clyde valley:—

“While the three main rivers resemble each other in thus breaking through a chain of hills to find their way into their firths, they present many points of difference in their respective courses across the lowland valley. Perhaps the most interesting is the Glyde. Drawing its waters from the very centre of the southern uplands, it flows transverse to the strike of the Silurian strata, until entering upon the rocks of the lowlands at Roberton it turns to the north-east, along a broad valley that skirts the base of Tinto. If the reader will glance at the map he will notice that at that part of its course the Clyde ap-proaclies within seven miles of the Tweed. Between the two streams, of course, lies the watershed of the country, the drainage flowing om the one side into the Atlantic, and on the other into the North Sea. Yet, instead of a ridge or hill the space between the rivers is the broad flat valley of Biggar, so little above the level of the Clyde that it would not cost much to send that river across into the Tweed. Indeed, some trouble is necessary to keep the former stream from eating through the loose sandy deposits that line the valley, and finding its way over into Tweeddale. That it once took that course, thus entering the sea at Berwick instead of at Dumbarton, is probable; and if some of the gravel mounds at Thankerton could be re-united it would do so again. Allusion has already been made to this singular part of the water-shed. Its origin is probably traceable to the recession of two valleys, and to the subsequent widening of the breach by atmospheric waste and the sea.

“From the western margin of the Biggar flat the Clyde turns to the north-west, flowing across a series of igneous rocks belonging to the Old Red Sandstone series. Its valley is there wide, and the ground rises gently on either side into low undulating hills. But after bending back upon itself, and receiving the Douglas Water, its banks begin to rise more steeply, until the river leaps over the linn at Bonnington into the long, narrow, and deep gorge in which the well-known falls are contained. That this defile has not been rent open by the concussion of an earthquake, but is really the work of sub-aerial denudation, may be ascertained by tracing the unbroken beds of lower Old Red Sandstone from side to side. Indeed, one could not choose a better place in which to study the process of waste, for he can examine the effects of rains, springs, and frosts in loosening the sandstone by means of the hundreds of joints that traverse the face of the long cliffs, and he can likewise follow in all their detail the results of the constant wear and tear of the brown river that keeps ever tumbling and foaming down the ravine.”