GRADUAL FILLING OF VEINS. 
609 
tions true pebbles of quartz and slate in a. tin lode of tbe 
Relistran Mine, at tbe depth of 600 feet below the surface. 
They were cemented by oxide of tin and bisulphuret of cop¬ 
per, and were traced over a space more than twelve feet long 
and as many wide.^ When different sets or systems of veins 
occur in the same country, those which are supposed to be 
of contemporaneous origin, and which are filled with the 
same kind of metals, often maintain a general parallelism of 
direction. Thus, for example, both the tin and copper veins 
in Cornwall run nearly east and west, while the lead veins 
run north and south; but there is no general law of direc¬ 
tion common to different mining districts. The parallelism 
of the veins is another reason for regarding them as ordinary 
fissures, for we observe that faults and trap dikes, admitted 
by all to be masses of melted matter which have filled rents, 
are often parallel. 
Fracture^ Re-opening and successive Formation of Veins .— 
Assuming, then, that veins are simply fissures in which chem¬ 
ical and mechanical deposits have accumulated, we may next 
consider the proofs of their having been filled gradually and 
often during successive enlargements. 
Werner observed, in a vein near Gersdorff, in Saxony, no 
less than thirteen beds of different minerals, arranged with the 
utmost regularity on each side of the central layer. This 
layer was formed of two plates of calcareous spar, which 
had evidently lined the opposite walls of a vertical cavi¬ 
ty. The thirteen beds followed each other in corresponding 
order, consisting of fluor-spar, heavy spar, galena, etc. In 
these cases the central mass has been last formed, and the 
two plates which coat the walls of the rent on each side are 
the oldest of all. If they consist of crystalline precipitates, 
they may be explained by supposing the fissure to have re¬ 
mained unaltered in its dimensions, while a series of changes 
occurred in the nature of the solutions which rose up from 
below : but such a mode of deposition, in the case of many 
successive and parallel layers, appears to be exceptional. 
If a vein-stone consist of crystalline matter, the points of 
the crystals are always turned inward, or towards the centre 
of the vein; in other words, they point in the direction 
where there was space for the development of the crystals. 
Thus each new layer receives the impression of the crystals 
of the preceding layer, and imprints its crystals on the one 
which follows, until at length the whole of the vein is filled: 
the two layers which meet dovetail the points of their crys¬ 
tals the one into the other. But in Cornwall, some lodes pc- 
* Came, Trans, of Geol. Soc. Cornwall, vol. iii., p. 238. 
26 * 
