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THEORY AND I’llACTICE OF ENGINEERING.

Book II.

To the civil engineer the geological map, with the heights of the sections above a certainlevel now forming, will be of great utility, enabling him to investigate the geologicalfeatures of particular districts. In many cases several portions of the same district havebeen examined and reported upon, and sections of the strata have been surveyed and laiddown, showing the position of the beds, to a scale of 40 feet to an inch : the details are givenwith full investigation into their mineral deposits, and notice taken of their organic remains.The geological structure is shown in different directions, to a scale of 6 inches to the mile,with the relative heights and distances. The maps are duly verified and coloured, toexhibit the detail with the boundaries of the coal fields and districts of different formations.The agricultural character of the country is defined, and for the mining districts contourlines distinguish the dip of the various beds of rock or coal, and as the faults in a districtare known, the means of ascertaining exactly where the vein is again to be met with areclearly given. These contour lines are highly useful, as they assist in affording informationon the subject of drawing off the water, which, if suffered to accumulate, would put a stopto mining operations. The miner and agriculturist are interested in the completion of thisgeneral survey, which will at once attest the importance of geology to all practical men.For the supply of water, the selection of building materials or the metals, this map promisesto afford a clear and useful guide, and to establish principles which will direct all operationswhere either the architect or engineer are employed.

The advances made of late years in geological knowledge have unquestionably producedgreater certainty in all mining operations. To obtain a mineral ore deposited in veins,different means must be adopted to those required when it is in alternate beds : and it israre to find the process the same in any two districts. In the tin and copper mines, for in-stance, in the west of England, the ore is drawn from fissures in the carboniferous or newerPahcozoie system, or from those cracks in igneous rocks which crop out near the surface,and such minerals are often of great value. In Sweden the ore is obtained in large lumps,and used for the manufacture of the best quality of steel; differing materially from thatregularly bedded in the stratification, and constituting a part of the fossiliferous stratifiedrocks.

Ferns usually traverse the strata in a direction by no means agreeing with, or having anyreference to, their deposition ; they have more the resemblance of fissures or cracks, whichhave been subsequently made in the rocks, and have by some mechanical means been filledup with mineral substances, which are generally, in such cases, either copper, lead, tin, orother metals, united with sulphur, carbonic acid, and other bodies, with argillaceous orsiliceous substances ; and such veins usually pass through the strata of rocks in a down-ward direction, often beyond the reach of the miner ; they are also found extendinghorizontally, and diminishing in their course until absolutely lost. They differ in nameaccording to the district: they are called pipe-veins when their direction is nearly parallelwith the beds and the ore makes no shoots in a lode; where shoots occur, the vein dipsfrom the neighbouring granite, which is the case whether the rock containing the ore bein itself crystalline or the effect of stratification. Shoot-veins implies a state wheremasses of ore occur in stratified rocks, and take an oblique direction, conformable to thedip of the beds.

Fluts is another term given to those lateral extensions which are parallel to stratification,as in those veins which spread over layers of any particular rock. There are other termsexpressing the condition in which the ores are met with, as that made use of in Germany ,called stockwork, which indicates the working in floors one below the other.

Feins are found in a vertical position, and seldom at a greater angle than 10°, though inCornwall they are met with occasionally at 45° ; they affect the mineral character of therocks that contain them by softening them, probably by the water, which circulates morefreely around and within, them. Hie dip or inclination of a lode is called the slope, andthe intersection of the vein with the surface the strike ; and in a mine the terms applied toa house are used, the floor, the roof, and the walls. The ores found in the metalliferousveins have no relatioft whatever to the rocks through which these run, the contents of a veinbeing silex, fluor spar, or carbonate of lime, which the miners call vein-stone, in which isthe ore from whence the metal is extracted; and the more intelligent the geologist, the lessdifficulty will he have in deciding whether a vein will be productive, and deserving thelabour of the miner.

Crystalline quartz, fluor spar, and carbonate of lime, contain ore in one form or another,either in small veins, or in crystalline masses, or disseminated crystals. At other times arich ore is discovered in an earthy and powdery state; when the fissures are filled with clayor sandstone, they rarely abound with ore in a sufficient quantity to make it worth pre-serving. A change in the nature of the rock indicates to the miner what he is to expect forhis toil, there being in most districts an order in the arrangement of the ores in veins. Inthe older rocks, when a vein contains copper, and it passes from a slaty formation, withoutaltering its direction, into granite, it at first increases in richness, and afterward?; gradually