The formation of sediments

In mountains running water makes deep valleys in the solid rocks. Rivers carry the mud to the sea. On reaching the sea the coarse gravel is usually deposited near the mouths off the rivers, while the finer material is carried further on. In lakes, the material brought down by the streams likewise settles on the bottom.

Besides the material, which surface waters carry in suspension, as mud, sand or gravel, they contain substances in solution. In limestone districts, for example, the waters contain lime. Besides lime other minerals are chemically precipitated in ocean and lake waters, especially silica, but all in a far less degree. Rivers bring continually to the sea large quantities of mineral matter, both in suspension i and solution.

Altogether, from all the rivers of the world it has bet it estimated that annually 2,735,000,000 tons of mineral substances are added, in solutions, to the oceans.

The upward and downward movements of the land at various points may bring sediments out of water and they may become part of the land. They harden with time. It is possible |to find sedimentary rocks on the top of mountains as well as in the low plains.

The ordinary sedimentary rocks are conglomerates, grit, sandstone, quartzite, shale slate, limestone, marble, and dolomite. Beds of pebbles, when cemented together, form conglomerate. A coarse and impure sand, when hardened, is called a grit; a coarse impure sand, usually derived from granite, is called arkose.

 

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ORE MINERALS

 Most of. the metals are deposited in the deeper regions mainly as sulphides. Metals may also be precipitated in the native form, as is the case with gold, platinum, and copper; they mау form rarer combinations, such as arsenides, tellurides, etc., or they may be deposited as oxides.

We generally think of the formation of oxides as characteristic of the surface, and of the sulphides as the natural products of the deeper regions. This is true, as a rule, for sulphides commonly change into oxides during the process of weathering. But the opposite extremes of conditions from those prevalent near the surface, namely, considerable heat and high pressure, and the presence of strong solutions and vapours may also produce oxides.

An oxide of iron, hematite, is often deposited from gases in volcanoes. Magnetite and hematite are also found in contact deposits, formed by the highly concentrated, heated and compressed solutions and vapours given off from cooling masses of magma.

Within slowly solidifying rock magmas, oxides of iron (magnetite and titaniferous magnetite), and probably oxides of iron and chromium (chromite, chrome iron), etc., are ac­cumulated to such an extent as to form orebodies. These oxide orebodies may also occur on or near the contact with the igneous rock; or near by, in the intruded rock. Since their temperature of consolidation is nearly that of the igneous rocks, they are more closely associated with them than are the sulphide ores.

 

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AERIAL PHOTOGRAPHY

III prospecting virgin coal-fields, aerial photography will, in future, play an important part. An unexplored unmapped region can be photographed rapidly from the air and a topographic map can be built up by examination of stereoscopic pairs. It is frequently possible to make deductions concerning the.geology of an area directly from the aerial photographs and, in this way, a great deal of valuable information may be, gained before ground parties set out to map the new terrain.

As both geological maps and aerial photographs are horizontal projections, they are amenable to the same interpretative projections and to the same interpretative, treatment. It is therefore necessary to have an understanding of the interpretation of geological maps before a study of aerial photographs is undertaken. There are two ways of interpreting geological maps, the first by relating the geological contacts to the topographic contours, and the (second by studying the outcrop patterns. The latter method, is of most use in studying large-scale reconnaissance maps but both methods are needed for detailed maps.

Under favourable conditions dip and strike of beds, folding, faulting and unconformity may be detected and some-conclusions may be reached regarding the nature of various rocks. In areas where the rocks are largely exposed, differenti­ation between the various rock types is greatly simplified. In most parts of the world, however, there is some superficial cover on the bedrock. 

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THE ORIGIN OF COAL

The territory of the present Donbass was once a bay of this sea. Giant plants and trees grew on its shores. These plants and trees fell into the marsh of the bay and were covered by sand and silt. The compressed deposits of trees and other plants underwent decomposition, and deposits of coal were formed.

Coal is a sedimentary rock of organic origin and is usually described as. a mass of mummified plants. Several fossil fuels are included under the word "coal": brown coal, lignite, bituminous coal, anthracite. Peat, though the first stage in the formation of most coals, has never been included under that name.

Practically all black coals show banding of soft bright and hard dull coal ("brights" and "hards") parallel to the bedding and often on the bedding planes, fragments of a substance rather like charcoal—"mineral charcoal".

The banded coals were divided into four constituents, vitrain, clarain, durain and fusain. The first two make up the "brights", the durain is the "hards" and the fusain is the "mineral charcoal"; Examination with the microscope has shown that the bright vitrain streaks represent large pieces of wood or bark with their cells filled with a bright black substance originally introduced as a jelly; clarain is similar to vitrain, but made of plant debris such as leaves and twigs, and fusain is partly carbonized wood. Durain is composed of more highly carbonized material with a high proportion of sporecoats, both megaspores and microspores. The "hards" and "brights" are generally concentrated at more or less definite levels in a seam, though they are sometimes inter-banded.

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THE SEARCH FOR COAL

The, science of geology plays an important part in the discovery and determination of the extent and value of coal deposits. Formerly coal was sought for in almost every geological formation, and much money was spent in useless boring and sinking operations. Now, almost in all countries of the world, the coal-fields have been geologically surveyed, and most of them accurately mapped in great detail, so that the mining engineer and others are able to determine what seams of coal, beds of ironstone, and fire clay are likely to exist under any given area.

It is also possible to know what succession of strata is likely to be penetrated if boring operations are to be carried. out.

There yet remain, however, some parts of the world which the mining engineer will have to carry out his own geological investigation and mapping. The search for coal, therefore, is differently commenced according to the nature of the information available with regard to4 the locality in which the exploration is to be made.

In recent years there have been witnessed5 amazing achievements in the realms of science. The properties of electrical conductivity or resistivity, magnetic permeability, density, elasticity and radioactivity, are all used by geologists in the search for coal and other minerals.

The electrical resistivity method of geological surveying depends upon the difference in physical properties exhibited by rock formations. Thus, the electrical resistivity method has been applied extensively and proved of value in the location of concealed faults, dykes, different types of rock contacts and mineral veins, etc., at shallow and moderate depths.

 

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