THEORY AND PRACTICE OF ENGINEERING.

Book I.

6 £ 0

In the formation of a railroad or a canal, a vast advantage* is gained by an acquaintancewith the structure of the land to be operated upon. The setting out of the line dependsupon the geological character of the country ; and in carrying the work into execution,much labour and expense may be saved, by understanding how the various soils passedover should be worked.

The clay, which alternates with occasional sand when deeply cut, is often dangerous,and is maintained in its position with great difficulty ; the water carrying away particlesof sand, leaves the clay to slide down upon its slippery bed after the layer of sand iswashed away. Wherever clay is cut through in the direction of its line of strike, it hasa tendency to slip, which can only be avoided by draining or drawing otF the water of eachbed cut through, or by giving a greater inclination to the rise side of the bed than to thedip side.

Chalk offers but little difficulty, as it will stand nearly perpendicular, but it is preferable,on account of the drainage, to make the cutting through it at right angles to the strike ofthe beds. Where sandy or loamy earth occurs, the slope of the excavations should alwaysbe made, if possible, in conformity with the dip of the bed, and so in other loose soils.

In tunnelling the geologist is the best pioneer; he alone can point out the state of thevarious strata and the nature of the rock to be cut through, the probable amount of watercontained by them, and the best method of draining it off.

Even in making and repairing the common highways, a scientific acquaintance with thesubject will render the selection of the materials a much more secure and easy bisk ; it toofrequently occurs that no consideration is made of either the friction or pressure; it is suffi-cient that the ruts are filled up, whether usefully and durably is not considered.

The ordinary Hint when used should be broken to a regular and moderate size ; lime-stone forms a smooth, but not so durable a road ; whinstonc, where it can be obtained, ispreferable to either.

In the formation of harbours, or the works which appertain to them, where the violenceof the ocean is to be resisted, it is impossible, without a study of the coast, to effect anythingthat shall be permanent, except by experiments causing an unnecessary outlay. Thegeologist observes the forces by which the projecting headlands are gradually worn away,and the earthy matter deposited by the various currents, and is thus enabled to direct thebest means of resisting the injuries to which most harbours are subject, and providingagainst their silting up, and the formation of bars or other impediments to the vesselsseeking shelter.

In the selection of materials for the purposes of building, geology has been of greatassistance, pointing out to us that a stone which resists the action of the air may be de-composed by water, and one that is porous and impermeable to water may be disintegratedby frost or the action of the atmosphere.

Limestones are divided into argillaceous, crystalline, and another variety, which composethe several Oolites. The Magnesian Limestones require great care when used for buildingpurposes ; they are obtained from the beds which overlie the lower new red sandstone, fromwhich they are separated by the coal measures.

The sandstones employed in building are of various qualities, those taken from the olderrocks are mostly preferred. The Craigleith is of excellent quality, of a light grey colour,and a fine grain; it contains 98 per cent, of silica, and not more than one per cent, of car-bonate of lime; it is heavy, and its particles are firmly bound together ; the beds are insome instances 10 feet in thickness ; it is much more difficult to work than Portland stone.There are many varieties of gritstone, some where quartz grains are united with argilio-siliccous cement, but those containing iron arc subject to decomposition, and they absorbupwards of a fourth part of their bulk of water. All stone should be applied in a buildingas it is found in the natural bed, for being usually laminated it is liable to split, if loadedor pressed in a direction opposite to the planes of deposition.

As this subject is more fully considered in the sequel, we shall proceed with someaccount of the different strata in the order they appear, commencing at the earth’s surface,and noticing the organic bodies that are found in the several beds, for the purpose ofidentifying the strata. This order and arrangement belongs more immediately to thepalaeontologist: a brief account therefore of the various classes and orders of these organicremains will be sufficient.

Organic remains or fossils, which are found in every stratified formation, are numerous,and endless in their variety ; they consist of both animal and vegetable productions, and thestudy of them is termed Palaeontology , or a description of what had existence some agespast. The animals found are divided into the Vertebrated and Invertcbrated.

The^/jrstf form four classes, Mammalia , Birds, Reptiles, and Fishes.

The yfammulia are divided into nine orders ; first , Alan, or Bimana ; second , Quadrumana ;third, Carnivora , which is subdivided into four families, the Cheroptera, the Insectivora , theriantigrada and Digitigrada, and the Amphibia; fourth order, the Marsupials ; fifth, the