Geology.--The geology of Renfrewshire claims special attention, on account of the remarkable development of volcanic rocks belonging to the Lower Carboniferous period, and the important series of coal-fields situated to the N of the volcanic area between Houston and the E border of the county near Rutherglen.
     The various subdivisions of the Carboniferous system are represented within the limits of the county. Beginning with the red sandstones lying at the base of this formation, which are the oldest strata in Renfrewshire, they occupy a belt of ground along the coast in the neighbourhood of Innerkip. They are merely the prolongation towards the N of similar red sandstones fringing the Ayrshire coast between Ardrossan and Largs. Consisting mainly of red sandstones and cornstones with bands of breccia and conglomerate, there is little variety in the character of the strata. They stretch inland, from the shores of the Firth of Clyde at Innerkip to the hills near Loch Thom, where they are thrown into a gentle anticlinal fold, succeeded by the overlying Cement-stone series, of which, however, there is but a limited development. Throughout Renfrewshire the Cement-stone series is almost wholly represented by a prodigious succession of contemporaneous volcanic rocks, which are the continuation of the great volcanic belt on the N side of the Clyde, forming the Kilpatrick Hills. There can be little doubt of the precise geological position of these volcanic rocks in this county, because, to the W of Loch Thom, they rest conformably on the white sandstones and Cement-stones, and where no faults intervene they graduate upwards into the Carboniferous Limestone series. They form a belt of hilly ground stretching across the county in a NW and SE direction, from the hills S of Greenock, by the Gleniffer Braes, to the high grounds round Eaglesham. In the E portion, the volcanic rocks form a low anticlinal arch, the axis of which coincides generally with the trend of the chain, the overlying strata being inclined towards the SW and NE. Throughout this extensive area the igneous rocks consist of basalts, melaphyres, and porphyrites, with intercalations of tuffs and coarse volcanic breccias. The upper and under surfaces of the lava flows are extremely slaggy and scoriaceous, and the cavities are filled with agates and various zeolites. The discharge of lavas and tuffs was so persistent that there are but few traces of sedimentary deposits in the volcanic series. In the neighbourhood of Eaglesham, however, sandstones, dark shales, and sometimes impure fossiliferous limestones are associated with the tuffs. An interesting feature connected with this remarkable volcanic area is the existence of numerous vents, from which the igneous materials were discharged. They are now filled with basalt, porphyrite, or volcanic agglomerate. The best example of one of these ancient cones is to be found on the hills between Queenside Muir and Misty Law, where there is a great development of coarse agglomerate pierced by dykes and bosses of felstone and basalt. This agglomerate pierces the stratified volcanic rocks of the district.
     As already indicated, there is a perfect passage from the contemporaneous volcanic rocks into the overlying Carboniferous Limestone series. The junction between the two, however, is usually a faulted one, and hence the regular succession is visible only at few localities. Where no faults intervene, the strata immediately overlying the ancient lavas consist of ashy sandstones, grits, and conglomerates, which are replaced at intervals by white sandstones and clay ironstones. Occasionally they are associated with bands of tuff. From the ashy character of the strata one might infer that the sedimentary materials were mainly derived from the denudation of the underlying volcanic rocks, while the bands of tuff indicate spasmodic outbursts of volcanic activity. The ashy strata just described are succeeded by the lowest members of the Carboniferous Limestone series. In Renfrewshire this important series of strata is divisible into three groups, in common with other areas in the midland counties, viz., a lower limestone group, a middle coal-bearing group, and an upper limestone group.
     Along the N border of the volcanic area, between the White Cart Water at Busby and the banks of the Clyde near Erskine House, the members of the Carboniferous Limestone series are everywhere brought into contact with the ancient lavas and ashes by faults. A glance at the Geological Survey maps (sheets 30 and 22 of the 1-inch map of Scotland) shows the irregular nature of the boundary line due to the peculiar system of faulting. In one remarkable case the Carboniferous Limestone series stretches almost continuously across the volcanic belt, from Johnstone and Howwood to Lochwinnoch. This hollow is flanked by two powerful faults, throwing down the lowest members of the overlying series.