SWELLING OE VEINS. 
611 
first set of deposits were cracked and often shattered, so that 
the new rent was filled, not only with angular fragments of the 
adjoining rocks, but with pieces of the older vein-stones. Pol¬ 
ished and striated surfaces on the sides or in the contents of 
the vein also attest the reality of these movements. A new 
period of repose then ensued, during which various sulphurets 
were introduced, together with hornstone quartz, by which 
angular.fragments of the older quartz before mentioned were 
cemented into a breccia. This period was followed by other 
dilatations of the same veins, and the introduction of other 
sets of mineral deposits, as well as of pebbles of the basaltic 
lavas of Auvergne, derived from superficial alluviums, prob¬ 
ably of Miocene or even Older Pliocene date. Such repeated 
enlargement and re-opening of veins might have been antici¬ 
pated, if we adopt the theory of fissures, and reflect how few 
of them have ever been sealed up entirely, and that a country 
with fissures only partially filled must naturally offer much 
feebler resistance along the old lines of fracture than any¬ 
where else. 
Cause of alternate Contraction and Swelling of Veins. —A 
large proportion of metalliferous veins have their opposite 
walls nearly parallel, and sometimes over a wide extent of 
country. There is a fine example of this in the celebrated 
vein of Andreasburg in the Hartz, which has been worked 
for a depth of 500 yards perpendicularly, and 200 horizontal¬ 
ly, retaining almost everywhere a width of three feet. But 
many lodes in Cornwall and elsewhere are extremely vari¬ 
able in size, being one or two inches in one part, and then 
eight or ten feet in another, at the distance of a few fathoms, 
and then again narrowing as before. Such alternate swell¬ 
ing and contraction is so often characteristic as to require 
explanation. The walls of fissures in general, observes Sir 
H. De la Beche, are rarely perfect planes throughout their 
entire course, nor could we well expect them to be so, since 
they commonly pass through rocks of unequal hardness and 
different mineral composition. If, therefore, the opposite 
sides of such irregular fissures slide upon each other, that is 
to say, if there be a fault, as in the case of so many mineral 
veins, the parallelism of the opposite walls is at once entire¬ 
ly destroyed, as will be readily seen by studying the annex¬ 
ed diagrams. 
Let a 5, Fig. 633, be a line of fracture traversing a rock, 
and let a Fig. 634, represent the same line. Now, if we cut 
in two a piece of paper representing this line, and then move 
the lower portion of this cut paper sideways from a to a, tak¬ 
ing care that the two pieces of paper still touch each other at 
