by William Battrum
SKETCH OF THE GEOLOGY OF THE DISTRICT.
BY A MEMBER OF THE GLASGOW GEOLOGICAL SOCIETY.

The western and north-western tracts of the county are composed of rocks that belong to the Silurian system, and are supposed to be the equivalents of the fossiliferous Silurian rocks of the south of Scotland, and which exist in the northwest Highlands in a much higher crystalline or metamorphosed condition than they are found in the southern tracts of this country. The principal varieties of these old crystalline stratified rocks, found in this county, are rocks of a gneissage structure, passing into mica, chlorite, talc, and clay slates, and which are sometimes associated with beds of quartz rock and crystalline limestone. These rocks often rise into hills of considerable altitude, and form the highest points of elevation in this part of the country. Their rugged peaks, wild ravines, and coast sections, impress their features on the scenery over nearly every part of the west Highlands. They are often pierced or cut through by veins or dykes of felstone porphyry and other intrusive igneous rocks, and in many parts of the district, (especially about Luss,) the schistoze rocks are very much contorted and twisted. Crystals of quartz and other siliceous minerals, besides metallic ores, are occasionally found in this group of rocks. But no organic remains have yet been found; their absence is attributed to the highly metamorphosed condition of the rocks. If any organic remains originally existed in them, they seem all to have been destroyed. The red sandstone of this country forms part of that belt of old red sandstone which stretches across Scotland from sea to sea, along the whole front of the Grampians, and rests unconformably upon the older crystalline schists already noticed. It forms the whole of the rock along the coast between Helensburgh and Dumbarton, and also forms the tract of land which stretches up the valley of the Leven, the islands in the lower reaches of Lochlomond being also composed of it. Its prevailing colour is bright red, and it affords a good, durable building stone in many localities. It belongs to what is termed the middle division of the old red sandstone, and in the west of Scotland it has yielded as yet few recognisable organic remains.

Resting conformable upon the old red sandstone, there occurs near Dumbarton, on the east side of the valley of Leven, an interesting group of thin bedded rocks, known to geologists as the Levenside limestones; they are composed of thin bands of nodular limestone, interstratified with a dark gray marly shale, which crumbles down rapidly on exposure to the weather. This formation is capped by beds of white coloured sandstone, which is overlaid by the trap rock which forms the higher parts of the Kilpatrick hills. The best sections of these thin bedded rocks are to be seen in Auchenreoch glen, on the Levenside grounds, where they form very lofty eminences on each side of the glen, and present to the geologist some of the finest veins of stratification to be seen in this country side. The strata is traversed by several intrusive dykes of greenstone and felstone, which being harder than the surrounding beds, often stand up as natural walls across the glen. Between partings of the strata are to be found, at one or two points, some thin veins of a fine, white, crystallised gypsum, from which fine specimens are to be obtained. The limestones seem never to have been worked for any economic purposes, and the only organic remains yet found in these beds are some obscure fragments of plants and scales of fishes, which occur in one of the gray sandstones near the base of the group. At present these beds are disputed among geologists, as to whether they belong to the upper old red sandstones or to the lower coal series, fossil evidence being wanted to enable them to be linked to either group. The rocks which form the eastern part of the county of Dumbarton belong to the Carboniferous system. The long ridge of the Kilpatrick hills bounds the coal-field to the north. This group of trappean heights, which terminate in Dumbarton Castle rock, and the heights above Bowling, belong to that chain of trap hills of volcanic origin which run across Scotland, from Ardrossan on the south-west to near Montrose on the north-east. The Kilpatrick division of the range has long been famous among mineralogists for the fine series of zeolitic minerals found in veins, &c., of the rocks. In the Bowling quarry and the rocks of the Long Craigs, very fine specimens of Prehnite, Thomsonite, Stilbite, Newlandite, and other minerals of the same group are to be found, and are much sought after by collectors. The features impressed by these trap hills upon the surrounding scenery are often very fine, and in few localities do they lend such a charm as on the banks of our own noble river in the neighbourhood of Bowling where the lofty and well-marked terraces of trap set high upon the hillside are surrounded at their base by the natural watchtowers of Dumbarton and Dumbuck, and other small eminences, the whole imprinting such a charm on the landscape that, when once seen in all its beauty, is not easily thereafter effaced from the memory.

    


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