Pan TX. §1.] MINERAL VEINS. 595 
_ to the opening which allowed the deposit of the silica, or was 
- introduced into a fissure opened between 2 and 38 after the deposit 
of the silica, is uncertain." 
. The occurrence of rounded pebbles of slate, quartz, an i 
in the lodes of Cornwall at depts of 600 feet fie the ae 
eneiss in the vein at Joachimsthal at 1150 feet, and of Liassic inna, 
and fresh-water shells at 270 feet in veins traversing the Carboni- 
ferous Limestone of the Mendip Hills and South Wales, seems to 
indicate that fissures may remain sufficiently open to allow of the 
introduction of water-worn stones and terrestrial organisms from the 
surface even down to considerable depths.? 
Connection of veins with faults and cross veins.—While 
any divisional planes in rocks may serve as the receptacle of mineral 
depositions, the largest and most continuous veins have for the most 
part been formed in lines of fault. These may be traced sometimes 
in a nearly straight course for many miles across a country, and as 
far downward as mining operations have been able to descend. 
Sometimes veins are themselves faulted and crossed by other veins 
Like ordinary faults also, they are apt to split up at their termina- 
tions. These features are well exhibited in some of the mining 
districts of Cornwall (Fig. 306). 

fi 
1 

T = 
j ill ie 
i we Be 
u 
ww! ww! 
yuil nnn 
awe Pe 
pil e will fa 
Fic. 306.—PLAN OF WHEAL FortuNnE Lopr, ConnwaLL (B.). 
_ Lim, lodes, of which the main one splits up towards east and west, traversing elvan 
dykes, e e, but cut by faults or cross courses, dd. Scale one inch to a mile, 
}! 
fun 
The intersections of mineral veins do not always at once betray 
which is the older series. If a vein has really been shifted by 
another, it must of course be older than the latter. But the evidence 
of displacement may be deceptive. In such a section as that in 
Fig. 307, for example, a cursory examination might suggest the 
inference that the vein d e must be later than the dyke or vein a b 
by which its course appears to have been shifted. Should more 
careful scrutiny, however, lead to the detection of the vein crossing 
the supposed later mass at ¢, it would be clear that this inference 
must be incorrect.” In mineral districts different series or systems 
of mineral veins can generally be traced, one crossing another 
belonging to different periods, and not infrequently filled with 
1 De la Beche, Op. cit. p. 699. 
2 De la Beche, Op. cit. p. 696. Moore, Q. J. Geol. Soc. xxiii, 483; Brit. Assoc. 
| 1869, p. 360. 
4 3 De la Beche, Op. cit. p. 657. 

2 OE a 

