On Mineral Veins. 235 
the gradual formation of the veins by the same law of 
replacement atom by atom, which causes a tendency in the 
constituents of some rocks to aggregate in bands or round 
some centre—a law still obscure, but the existence of which 
is now generally acknowledged. The forming of the vein 
fissures and the deposit of minerals in them appear to have 
been simultaneous operations. 
It will also be found that the majority of mineral veins 
are not formed on faults, but either on the cleavage or divi- 
sional planes. 
All rocks, however elevated they may be above the water 
level of the country, are more or less porous, and hold a large 
per centage of water. 
The most compact marbles known will hold , part, and 
ordinary limestones = part of their weight of water, while 
Bath stone will take up one gallon, and chalk two gallons 
per cubic foot. Mr. 8. T. Hunt has carried out a series of 
experiments to ascertain the amount of water that can be 
absorbed by the paleozoic rocks of Canada. He found that 
the limestones and sandstones would take up from 1 to 13 
per cent., and the shales from ‘75 to 7-94 per cent. of their — 
weight of water. A bed of rock absorbing only 2°5 per cent., 
and one hundred feet thick, would contain 70,000,000 cubic 
feet of water in a square mile of area, a quantity sufficient 
to supply seven gallons per minute for over thirteen years. 
This water is never pure, for it invariably holds different 
mineral substances in solution in greater or less quantity, 
which must excite slow electro-chemical action, intensified 
by the magnetic currents constantly circulating in the crust 
of the earth. However unchangeable the rocks may appear 
to our limited experience, it is certain that these forces acting 
during long ages have caused, and are now slowly causing, 
great changes. Consolidation of the particles of the rocks, 
alterations of their crystalline structure, aggregation of their 
constituents into nodules or bands, decomposition under a 
change of conditions of the bodies previously formed, are all 
forces which have been in operation from the time when the 
rocks were first deposited, and are now probably as active 
as ever. The views as to the great change thus produced in 
the structure of rocks are constantly being extended. Nearl 
thirty years ago the late Mr. Evan Hopkins called attention 
to instances of the gradual change of granite to gniess, and 
gniess to mica-schist ; and from these drew the conclusion 
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