Economic Geology

ALL the minerals and metals with which the arts and manufactures have to deal being obtained from the earth's crust, some knowledge of its structure and composition is indispensable to the economic geologist. For this reason we devote the present chapter to a brief outline of Geology; more especially as regards the physical characters of rocks and minerals, their modes of occurrence, and their chronological arrangement. With due attention any intelligent reader may easily make himself acquainted with these peculiarities; and the more intimate his knowledge, the better will he be enabled to understand the nature of the industrial products and processes that may come under review.

Stratified and Unstratified Rocks.

The exterior crust, which forms the theme of the geologist, is composed of rocks; and under this term are included all its substances, whether hard or soft, superficial or deep-seated -sands, sandstones, clays, shales, peats, coals, limestones, ironstones, lavas, basalts, granites. Whatever their mineral character, these rocks are found to occur in two main positions -stratified or bedded, and unstratified or eruptive. Reasoning from the manner in which rock-matter is deposited at the present day in lakes, estuaries, and seas, the stratified are regarded as of sedimentary or aqueous origin—that is, as having been formed through and by the agency of water. And reasoning, in like manner, from the ejectments of volcanoes, the unstratified are regarded as of eruptive or igneous originthat is, as having been formed through and by the agency of fire. In the accompanying illustration, the "Castle Rock" of Edinburgh is a truly eruptive or unstratified mass breaking through the sedimentary or stratified sandstones and shales

which are tilted up, and slope away from the centre of eruption. Wind-blown materials, as sand-dunes-chemical deposits, as calcareous tufa-and organic growths, like peat-moss and shell-beds are usually classed with the stratified; while showers of volcanic ashes, and other irregular ejectments, though arranged more or less in layers, are described merely as stratiform. Generally speaking, the sedimentary rocks are formed from the waste and debris of pre-existing rocks, are lam

Edinburgh Castle Rock :-Basaltic Clinkstone passing through Lower Carboniferous Shales and Sandstones.

inated or bedded in structure, comparatively soft and fragmentary in texture, and frequently imbed the remains of plants and animals. The eruptive rocks, on the other hand, however originating, make their appearance from below, are amorphous, or occasionally columnar, in structure, uniform and crystalline in texture, and rarely imbed any traces of organic remains. While the preceding are the general characteristics of the stratified and unstratified rocks, it must be borne in mind that there are many anomalous masses of conglomerate and breccia on the one hand, and curious sheet-like overflows and stratiform ash-beds on the other.

At the present day, stratified rocks are being laid down in all lakes, estuaries, and seas, and unstratified ejected from all volcanic centres. And as the forces (meteoric, aqueous, organic, chemical, and igneous) by which old rocks are wasted and new ones reconstructed from their debris, are as enduring as the planetary system from which they take their rise, the geologist is entitled to ascribe the formation of the rocky crust to the operation of similar agencies in former periods. In this

way, land and water are gradually but continually changing places the rock-matter formed during each terraqueous change being not only the record of these mutations, but an indication of the physical aspects of our globe at the successive stages of its history.

Relative Positions of Rocks.

Laid down in water and assorted by water, the original position of the stratified rocks is that of horizontality; but having been subsequently acted upon by the vulcanic or eruptive forces, they usually occur, as may be seen in our sea-cliffs, ravines, and railway cuttings, in inclined, bent, and contorted positions, and more or less rent and fissured-some portions being thrown up and others thrown down, or, in technical language, faulted and dislocated. These rents and fissures are occasionally filled up with rubbly matter washed in from above, and sometimes with molten matter injected from below; and hence the occurrence of dykes, as they are termed "soft" in

a Simple fissure; b Fault; c Soft dyke; d Hard dyke; vv Veins.

the former instance, and "hard" in the latter-intersecting and interrupting the continuity of the sedimentary strata. Again, when these rents and fissures have been filled up by slow infiltration of mineral and metallic matter, they constitute veins and lodes-the veinstone or matrix consisting of calc-spar, fluor-spar, quartz, baryta, or other sparry material, while the accompanying metallic ores are in the state of oxides, sulphides, carbonates, and other chemical combinations. The slope at which a stratum lies to the horizon constitutes, in geological language, its dip or angle of inclination; that portion of a stratum which comes to the surface its outcrop or basset-edge; and a line at right angles to the dip its strike or stretch across the country-this strike being always at right angles to the dip, and vice versa.

Melted and erupted by vulcanic heat from below, the igne

ous rocks, on the other hand, occur as unstratified or amorphous masses, and as regards their relations to the strata through which they pass are spoken of as disrupting, or breaking through; overlying, having flowed over; interstratified, having flowed over and been subsequently covered by other sediments; and intrusive, when thrusting themselves, as it were, with some degree of parallelism, among and between the sedimentary beds.

Structure and Texture of Rocks.

The manner in which rocks are arranged or piled up in the crust constitutes their structure; and this structure is described by such terms as stratified, bedded, jointed, tabular, columnar, massive, amorphous, &c. The internal arrangement of their particles constitutes their texture, and this, as the case may be, is spoken of as earthy, granular, crystalline, fibrous, porous, compact, vitreous, &c. The columnar aspect of the basalt of the Giant's Causeway is its structure; a chip from any of the columns exhibits its internal crystalline texture. The outward portion of a rock, exposed to and acted upon by the atmosphere, is spoken of as its weathered surface; and the internal texture, laid open by the hammer, as its fresh-fracture. The fresh-fracture of a rock may give no indication how it will be affected by exposure to the weather; the weathered surface, on the other hand, exhibits faithfully the effect of meteoric agency in discolouring and disintegrating, and is consequently of great use to the builder and architect. The fracture of rocks depends on their texture, and is described as even, flat, bladed, hackly or irregular, splintery, conchoidal and subconchoidal, according to the appearance it presents. Roofing-slate, for example, splits up with a flat or regular surface, calcareous spar cleaves with an even or smooth face, while a piece of flint or cannel-coal breaks up with a conchoidal or shell-like fracture. A knowledge of the manner in which a rock breaks and cleaves is often of great use, not only in facilitating the operations of the quarryman and mason, but in preventing unnecessary waste of the material. A workmanquarrier, paviour, or mason-acquainted with the structure and texture of rocks, will not only turn out a larger amount of material with the same labour, but will, by his skilful manipulation, effect a saving of the material itself.

Hardness and Specific Gravity of Rocks.

The specific gravity of rocks is determined by the standard of distilled water at 60°, which is regarded as 1; and their

relative hardness is determined by the following scale, invented by the German mineralogist Mohs :

Thus, common hæmatite, or red oxide of iron—G = 4.5 − 5.5 ; H=5.5-6.5-means that its specific weight is from four and a half to five and a half times greater than that of water, and that in hardness it stands between five and a half and six and a half in the above scale. The determination of specific gravities is often a delicate and difficult operation; but the relative hardness of rocks and minerals is readily approximated. Thus, if a mineral scratches felspar, but is in turn scratched by rock-crystal, its hardness must be between 6 and 7, and may be indicated as 6.4 or 6.8, according as it seems to approach the felspar on the one hand, or the rock-crystal on the other. A knowledge of the relative hardness of rocks and minerals is often of essential importance in the arts and manufactures, and hence the value of their determination in practical Geology. The following table of Specific Gravities may also be of use for future reference :

Agate,