Embankments and Bridges.

In the matter of embankments, little geological knowledge is required beyond ascertaining the nature of the foundations on which they are to rest, the facility with which the banking material may be obtained, and the angle of repose at which such material is likely to remain. Embankments across marshes and peat-mosses are frequently of difficult execution, requiring not only a vast amount of material, but sometimes subsiding for months and years till the soft underlying bed be thoroughly squeezed out or compressed. Besides calculating so that excavation and embankment shall balance each other as nearly as possible, the side-slopes or angle of repose should be studied, blocks and hard rocky debris resting at inclinations at which sand, clay, and soft earthy substances could not possibly remain. Embankments are artificial rock-formations, and the more uniformly the material can be "tipped" and assorted, the more compact, impervious to water, and less liable to "sits" and slips, will the mound become.

As with embankments so with bridges; their foundations require similar attention, and though their construction be chiefly a matter of masonry, yet some geological acquaintance with the situation and the nature of the materials to be employed cannot fail to be of advantage. Whatever the nature of the bridge, whether stone or girder, everything depends upon a secure foundation; and this, again, depends on the geological nature of the material to be built upon, be it solid rock, tough firm clay, or soft silts requiring concrete, piles, or other device to secure stability. The alluvia in river-valleys are often of great depth and of treacherous nature; and it is frequently more judicious to make a deviation to secure a rocky foundation than to persevere in a straight line over such superficial accumulations.

Road-Materials.

Touching road-materials, a great variety of substances are used in Britain, but the best of them will not make a smooth and durable highway unless they be laid on a good bottoming of rubble-stones, and these again on a well-drained surface. On a wet soil the road-metal sinks, becomes uneven, and wears irregularly, while in winter the moisture freezes and expands, and when thaw comes the surface is broken up and the consistency of the metal destroyed. In all cases the scarf-skin of agricultural soil should be removed, and, if not needed for levelling up, can be readily disposed of on the adjacent farms for composts and admixture.

In some districts where granites abound they are broken for road-metal, but if large-grained and highly felspathic they are easily crushed and reduced to clay. In others por

phyries and felstones are employed, and many of these, from their hardness and toughness, stand well, though somewhat expensive in the breaking. In others, again, where traps prevail, greenstones and basalts are largely used, and when broken to proper size make by far the smoothest and most durable roadway. Of course there is great variety among these greenstones, but unless among the softer and more felspathic sorts, they are not only durable but cheaply procured. In some districts the harder pebbles (chiefly of quartz, porphyry, granite, &c.) from river-channels and the sea-shore are employed, in others the harder and more silicious limestones and sandstones; in some the flints from the chalk, and in others near blastfurnaces the slag is broken by crushing-machines, and makes a fair though not very durable metal. In all cases of macadamising, thorough drainage and a good bottoming of rough rubble is indispensable; and when the metal is laid on, a heavy rolling down and consolidation with some sharp gravelly binding is equally necessary. Basalts, greenstones, felstones, and felstone-porphyries, make the smoothest and most durable metal, and now, since the introduction of efficient crushing-machines, should be sought after by the road-maker, even though requiring to be brought from considerable distances.

In the construction of street thoroughfares subjected to heavy and continuous traffic, and where macadamising would be all mud in winter and dust in summer, recourse must be had to causewaying either in rubble or in regular courses. The former may do for by-streets where the traffic is light; but for the great public thoroughfares of a commercial town nothing will serve save coursed blocks of granite, porphyry, or greenstone. Wooden blocks, concrete, and asphalt are at their best but indifferent substitutes, and wholly unsuited where the gradients are steep or even considerable. Granites like those of Aberdeen, Argyle, Dalbeattie, Creetown, Wicklow, the Channel Islands, &c., are largely used in all the thoroughfares of our principal cities, and make clean and durable streets; but some kinds of porphyry, like that from the Moorfoot Hills, and employed in Edinburgh, are harder and tougher, though from their tendency to wear smooth they require to be laid down in narrower courses. Greenstones are also largely used, and when properly coursed and bedded are almost as durable as granite. Good samples of greenstone

causewaying may be seen in Edinburgh (Ratho, Dalmahoy Crags, and Corstorphine Hills), in Glasgow (Croy and Kilpatrick Hills), in Newcastle (Christon Bank and Wall), in Leeds (Richmond), and, indeed, in most of our large towns where the rock can be obtained at a cheaper rate than granite. In some instances quartzites and hard silicious sandstones are employed: they stand well, but, from their tendency to wear smooth and slippery, require to be laid, like the Moorfoot porphyry, in narrow courses.

For the footpaths of our towns flagstones are in great request, and some of them held in high estimation. Those from the lower old red sandstone of Caithness are extremely hard and durable, and can be obtained of great size, and from one to nine or ten inches in thickness. Being very hard and close in texture they are apt to become too smooth; some contain nodules of iron sulphide which resist wear, and render the surface irregular and somewhat dangerous, and many of the harder and thinner sorts are apt to crack unless carefully bedded, as all of them require to be. They are, however, impervious, or nearly so, to water, and make a clean and easily kept footpath. About 10,000 tons are annually exported from the county, at a value of between £4000 and £5000. Those procured from the lower old red of Forfarshire and Perthshire enjoy also a high reputation; but from their softer, more laminated, and absorbent nature, are better fitted for inside than for outside pavements. Local supplies of flagstones are also obtained from the coal-formation in several counties (Fife, Edinburgh, and Lanark); but those from the millstone grit and Gannister beds of Yorkshire and Derbyshire are perhaps the best and most durable, and capable of being raised of great size and of any thickness. Supplies are also obtained from the new red sandstone of Dumfriesshire and Cumberland, which are of fine even grain, and though rather soft, wear well and equably. Flags of fair quality are also raised from the middle oolite, and from the Wealden in Sussex, but they want the largeness, smoothness of surface, and compactness of texture which characterise those from the old and new red sandstones. What is wanted in a good flagstone is variety in thickness and size, a straight and even surface or bedding, and a non-absorbent and compact texture-laminated varieties being apt to split or peel off under the influence of frost and moisture. In some country towns the footpaths are neatly laid with pebble-stones; and in the suburbs of Edinburgh the larger chips obtained in dressing the causeway-blocks are closely

set on edge and beaten down to a smooth surface, thereby making a firm and durable material under the name of Hornising," after a burgh surveyor of the name of Horne.

For kerb-stones, granite, greenstone, and some of the harder sandstones and limestones have long been used. The granites of Aberdeen, Kirkcudbright, Wigtown, and Wicklow, make excellent material; some of the hornblendic greenstones, as those of Queensferry on the Forth, and of Corstorphine and Ratho near Edinburgh, are equally suitable, and more cheaply tooled; while many of the harder limestones (Derby, York, Westmoreland, and Devon) square well, and are extensively employed in their respective areas. In some towns sandstones on edge are employed; but these, unless hard and uniform in texture, are apt to split up under the weather, and wear irregularly. A straight, durable, and well-set kerbstone is indispensable to a good street, not only in retaining the flagstones and maintaining the gutters, but in imparting an appearance of finish and stability to all the lines and turnings.

II. RAILWAY CONSTRUCTION.

Many of the remarks on road-making are equally applicable to railway construction. The choice of route and study of gradients are determined in a great measure by the requirements of the district, though an engineer well acquainted with its geology and mineral resources may, with a little modification, not unfrequently add materially to the success of the undertaking. Where excavations and tunnellings are imperative, he will not only know better how to conduct these operations, but will be enabled to see how far the material excavated, or any portion of it, may be utilised on other portions of the line, either in walling, embanking, or building of bridges. From a study of the geological structure of the country passed through, he will also make provision for the development of its mineral resources, be these coals, metallic veins, building-stones, limestones, clays, or sands. The main line may not pass through all of these repositories, and yet it may be so planned that sidings and branch-lines can be conveniently attached in the event of subsequent requirement.

Cuttings and Tunnels.

In the matter of excavating, for example, whether in open cuttings or in tunnels, no proper estimate can be made of the expense without a knowledge of the nature of the rocks to be passed through. Homogeneous chalk rocks, limestones,

alternations of sandstones and shales, masses of hard pebbly conglomerate, of gneiss, of greenstone, or granite, are things altogether different, not only requiring different amounts of labour, but different appliances for their successful excavation. A cutting through tough diluvial clay thickly studded with blocks and boulders may be as expensive as a cutting through

Cutting through Boulder-Clay, Linlithgowshire.

sandstone; and while the latter may be utilised as a buildingstone, the former is only fit for the formation of an embankment. And even after excavation, one set of rocks-granites, greenstones, limestones, and compact sandstones-will resist the action of all weathers, and stand firm and secure, while another set-soft sandstones, shales, clays, and sands-will