8 SKETCH OF THE GEOLOGY OF DERBYSHIRE. 
right angles to the principal tension, and parallel to the axis of eleva- 
tion. That the fissures, likewise, would not begin at the surface, but 
at some point beneath it; and that those which are parallel must be 
necessarily contemporaneous in their origin. That subsequent move- 
ments would be likely to convert the longitudinal fissures into faults, 
producing considerable relative displacement of the beds on either 
side of them, and might cause the fissure to be very irregular -in its 
width at different points; that the width of the transverse fissures, on 
the contrary, would be likely to be more regular, and the relative 
movement of the beds on either side of them to be less. 
Now, applying these general results to the particular instance of 
Derbyshire, we know the axis of elevation of this district to be a 
north and south one, because it belongs to an elevated ridge which 
stretches away to the N. as far as the borders of Scotland ; and we 
have seen that the chief faults and most irregular veins have a N. and 
S. direction, while the E. and W. veins, on the contrary, are remark- 
ably regular. From all these circumstances it is clear that faults and 
veins are both due to a mechanical cause; that they are, in fact, both 
fissures produced by the same cause and at the same time, during the 
general elevation of the district. That a fault isa fissure generally 
devoid of minerals, on opposite sides of which the beds are relatively 
displaced ; that a vein is a fissure producing generally little displace- 
ment, but filled with minerals in a state of greater or less purity and 
crystallisation. In what way these mineral substances gained their 
present position, we must probably have recourse to the chemist to 
resolve. That they were not placed there by mechanical causes, as 
was at one time supposed, is certain ; because in Derbyshire the toad- 
stone almost invariably continues across the vein, completely cutting 
off the connection between the parts above and below. If, then, the 
veins had been mechanically filled from above, they ought to be empty 
below the toadstone; if from below, the part above the toadstone 
would contain no minerals. It is evident, moreover, from the veins 
of this and other districts, that the quantity and quality of the mi- 
nerals in a vein very greatly depend upon the kind of rock which 
composes its sides, making it probable that these minerals have been, 
in some way, segregated from the adjoming rock into their present 
position. This subject is, however, at present beset with difficulties. 
We come now to the next superior rock, 
