Part I. Sect. iv.§2.| WATER IN ROCKS. 299 
in proportion to their degree of porosity. Gypsum absorbs from 
about 0°50 to 1:50 per cent. of water by weight; granite, about 0:37 
per cent.; quartz from a vein in granite, 0°08; chalk, about 20-0; 
plastic clay, from 19:5 to 24:5. ‘These amounts may be increased by 
exhausting the air from the specimens and then immersing them in 
water.’ 
The interstitial water of igneous rocks may be either an original 
constituent, deriving its origin, like any of the component minerals, 
from molten reservoirs within the earth’s crust, or may have de- 
scended from the surface. Many facts may be adduced in support of 
the greater probability of the second view. Besides the general 
proximity of volcanic orifices to large sheets of water, we have 
abundant evidence of the actual descent of water from the surface, 
both through fissures, and also by permeation through the solid 
substance of rocks. All surface rocks contain water, and no mineral 
substance is strictly impervious to the passage of this liquid. The 
well-known artificial colouring of agates proves that even mineral 
substances apparently the most homogeneous and impervious can be 
traversed by liquids. In the series of experiments above (p. 263) 
referred to, Daubrée has illustrated the power possessed by water of 
penetrating rocks, in virtue of their porosity and capillarity, even 
against a considerable counter-pressure of vapour; and, without 
denying the presence of original water, he concludes that the 
interstitial water of igneous rocks may all have been derived by 
descent from the surface. 
The masterly researches of Poiseuille have shown that the rate of 
flow of liquids through capillaries is augmented by heat. He proves 
that water at a temperature of 45° C. in such situations moves nearly 
three times faster than at a temperature of 0° ©. At the high 
temperatures under which the water must exist at some depth within 
the crust, its power of penetrating the capillary interstices of rocks 
must be increased to such a degree as to enable it to become a 
powerful geological agent.’ 
Solvent power of water among rocks.—The presence of 
interstitial water must affect the chemical constitution of rocks. It 
is now well understood that there is probably no terrestrial substance 
which, under proper conditions, is not to some extent soluble in 
water. By an interesting series of experiments, made many years 
azo by Messrs. Rogers, it was ascertained that ordinary mineral 
constituents of rocks could be dissolved to an appreciable extent even 
by distilled water, and that the change was accelerated and augmented 
by the presence of carbonic acid. Water, as pure as it ever occurs 
in a natural state, can hold in solution appreciable proportions of 
1 See an interesting paper by Delesse, Bull. Soc. Géol. France, 2me sér. xix. (1861-2) 
p. 65. 
2 Comptes Rendus (1840), xi. p. 1048. Pfaff (Allgemeine Geologie, p. 141) concludes 
from his calculations as to the relations between pressure and tension that water may 
descend to any depth in fissures and remain in a fluid state even at high temperatures. 
3 American Journ. Science (2), v. p. 401. 
