" Parr I. Sucr.i.§ 5.) CAUSES OF VOLCANIC ACTION. 263 

force within. But Daubrée’s experiments have shown that, owing to 
eapillarity, water may permeate rocks against a high counter-pressure 
of steam on the further side, and that so long as the water is supplied, 
whether by minute fissures or through pores of the rocks, it may, 
under pressure of its own superincumbent column, make its way into 
highly heated regions. Experience in deep mines, however, rather 
goes to show that the permeation of water through the pores of rocks 
gets feebler as we descend. 
Reference may be made here to a theory of volcanic action in 
which the influence of terrestrial contraction as the grand source of 
volcanic energy has recently been insisted upon by Mr. Mallet He 
maintains that all the present manifestations of hypogene action are 
due directly to the more rapid contraction of the hotter internal mass 
of the earth and the consequent crushing in of the outer cooler shell. 
He points to the admitted difficulties in the way of connecting 
voleanic phenomena with the existence of internal lakes of liquid 
- matter, or of a central ocean of molten rock. Observations made by 
him, on the effects of the earthquake shocks accompanying the 
volcanic eruptions of Vesuvius and of Htna, showed that the fccus of 
_ disturbance could not be more than a few miles deep; that, in 
relation to the general mass of the globe, it was quite superficial, and: 
could not possibly have lain under a crust of 800 miles or upwards in 
thickness. The occurrence of volcanoes in lines, and especially along 
some of the great mountain-chains of the planet, is hkewise dwelt 
upon by him as a fact not satisfactorily explicable on any previous 
hypothesis of volcanic energy. But he contends that all these 
difficulties disappear when once the simple idea of cooling and 
contraction is adequately realized. “The secular cooling of the 
globe,’ he remarks, “ is always going on, though in a very slowly 
descending ratio. Contraction is therefore constantly providing a 
store of energy to be expended in crushing parts of the crust, and 
through that providing for the volcanic heat. But the crushing 
itself does not take place with uniformity; it necessarily acts per 
saltum after accumulated pressure has reached the necessary amount 
at a given point, where some of the pressed mass, unequally pressed 
as we must assume it, gives way, and is succeeded perhaps by a time 
of repose, or by the transfer of the crushing action elsewhere to some 
_ weaker point. Hence, though the magazine of volcanic energy Is 
_ . being constantly and steadily replenished by secular cooling, the 
_ effects are intermittent.” He offers an experimental proof of the 
sufficiency of the store of heat produced by this internal crushing to 
cause all the phenomena of existing volcanoes. The slight compar- 
1 Daubrée, Geologie Expérimentale, p. 274. See also Tschermak, Sitzber. Akad. Wien 
March 1877. Reyer, Beitrag zur Physik der Eruptionen, § I. 
. 2 Phil. Trans., 1873, _ See also Daubrée’s experimental determination of the quantity 
~ of heat evolved by the internal crushing of rocks. Géologie Hupérimentale, p. 448. 
$ The elaborate and careful experimental researches of this observer will reward 
attentive perusal. Mallet estimates from experiment the amount of heat given out by 
the crushing of different 10cks (syenite, granite, sandstone, slate, limestone), aud con- 

