which form the opposite Walls of Cross-veins. 379 
tersect and sometimes heave the lodes; but they do so only 
in certain parts of their descent, and not only disappear up- 
wards and downwards, but are also of very short lengths, for 
those which intersect one lode seldom extend to any other: 
certain cross-veins being, as it were, peculiar to some lodes, 
and not continuing far from them. 
(ff) At Polgooth, Reskilling Great elvan heaves both 
Saint Martin’s and Screed’s lodes, whilst Saint Martin’s lode 
intersects the Little elvan, which in every respect but its size 
resembles Reskilling Great elvan. 
gg.) The only other class of facts which seems to bear on 
this inquiry, but the evidence of which is of an inferential 
kind, is that comprising the heaves of the lodes at Balnoon 
and Dowgas, which take place without the intervention of a 
joint of rock*, or of a cross-vein, or any other intersecting body 
whatever. 
Let us now proceed to consider how far these heaves and 
intersections can be reconciled with any assumed motions; 
either on real planes, as those of the cross-veins, or on ima- 
ginary ones adopted for the sake of argument alone. 
If we grant that the formation of lodes was prior to that of 
cross-veins,— 
I. The intersection of a single lode by a single cross-vein, 
whether the result be a simple intersection, or a heave to the 
right-hand or to the left, is easy of solution. 
A simple intersection might be effected by the mere open- 
ing of a fissure, and the introduction into it of materials 
forming a cross-vein. Or if the lode were perfectly straight 
in its descent, the same result would be produced by the ele- 
vation of the portion of the lode (and of course of the con- 
taining rock also) on one side of the cross-vein, or by the sub- 
sidence of that on the other ;—provided that the motion took 
place in a direction parallel to the dip of the lode; as, for ex- 
ample, a vertical motion and a perpendicular lode. But lodes 
are never perfectly straight, but are curved and irregular when 
seen in profile. 
If, then, there be either an elevation of one side of a cross- 
vein, or a depression of the other, it would be a circumstance 
scarcely to be deemed accidental that the curvatures should 
occur in such regularly recurring and perfectly similar series, 
and that the degree of motion should be so adapted to them, 
that one of them should be raised, or the other lowered, so 
as to preserye a perfect coincidence with another, and thus 
retain the continuity which, in its primal state, had existed at 
a different leve]. Yet these coincidences (if either vertical or 
* Dr. Boase, Cornwall Geol. Trans. iv. p. 448. 
2C2 
