106 TECHNOLOGY. 
VII. MINING. 
Having in the preceding pages turned our attention more exclusively to 
machinery, we will now treat in separate sections of some branches of Prae- 
tical Technology, and particularly of the subjects of Mining, Smelting, and 
Agriculture. We will first take a brief view of Mrntne, as it relates to the 
extraction of the economical, or, as they are usually designated, the useful 
minerals. 
INTRODUCTION. 
All the useful minerals, in those parts of the surface of the earth to 
which we have access, are distributed into certain distinct groups. These 
minerals are distributed among other mineral substances, either in beds or 
veins. They occur either stratified or unstratified. The former are called 
layers, from the laminated structure they present, and beds. The term bed 
is principally applied to mineral coal, iron, &c. Layers or beds of minerals 
are sometimes horizontal, sometimes inclined at a considerable angle with the 
horizon, and sometimes distorted, bent, and broken. The want of stratifi- 
cation and a tendency to a crystalline structure show that the beds belong 
to an unstratified or massive formation. 
In observing a layer or bed of minerals, we notice first its strike or direc- 
tion, that is, the angle which it makes with the meridian line; its dip, or 
the angle which it makes with the horizon; the position and character of 
the hanging wall, or the rocks which bound the top of the bed, and of the 
foot walls, or those rocks which lie underneath the bed, the former being 
sometimes called the roof and the latter the floor of the bed; and finally 
the out-crop of the bed or its termination in the open air at the surface of 
the earth. Sometimes layers or beds which are horizontal for the greater 
part of their extent, rise up towards the out-crop and form basin or saddle- 
shaped folds. They are then called disturbed strata. We often find local 
dislocations and displacements of the beds, which are here and there inter- 
rupted by fissures which have since been filled up by some mineral sub- 
stance. These fissures run across all the strata of the formation. These 
interruptions in the continuity of strata in the same plane, accompanied by 
fissures, are called faults, which term is sometimes applied to the rock filling 
the fissure. These fissures are generally filled with basalt, or some similar 
rock ; the rock which fills the fissure is properly called a dyke. The com- 
plications produced by faults are very diverse; the mineral substance 
which constitutes the rock above and below it, and the fault which has 
caused the disturbance, being often mixed together, so that the character of 
the bed is materially changed. The faults often cause a separation and dis- 
location of the members of the formation. The thickness of the fault 
variés from one line to several yards. The strata separated by the 
fault have frequently suffered therefrom a change of place or a slide 
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