Notices of Memoirs — Causes of Volcanic Action. 33 



permits, they flash into steam and rise in vast bubbles of vapour to 

 the surface of the land. Of the quantity of this underground water 

 some notion may be formed by the fact that the deepest of the 

 three springs under Naples discharged, when first tapped, two cubic 

 metres (430 gallons) per minute. The water may pass in bodilj'' in 

 consequence of the powerful shocks and vibrations shaking and 

 breakino; the strata, and so causing masses of rock to fall in from 

 the sides of the main duct, accompanied by the water held in the 

 beds, or it may pass in by capillarity, for it is well known that 

 this state exercises a remarkable influence on the conditions of 

 equilibrium on the two sides of a porous body, and Mr. Daubree 

 has shown that water will pass through sandstone notwithstanding 

 the stronger resistance of steam. The experiments were only carried 

 to the extent of a stearn-pressure of two atmospheres, but it was 

 evident that the limits of the power were not reached. They 

 further also showed that heat materially increased the power. 

 There is reason to suppose that water, under the considerable 

 hydrostatic pressures that exist beneath volcanic mountains, and 

 assisted by capillarity, may flow into the volcanic ducts with facility, 

 especially when aided by the intermittent relief of pressure, afforded 

 by the rise and fall, or pulsations, of the column of lava. 



As the underground springs fed by the rainfall are exhausted by 

 the expulsion of the large volumes of water converted into steam, 

 another agent comes into operation. The surplus water, one portion 

 of which usually goes to feed the surface springs, while another 

 portion passes through the permeable strata into the sea-bed, is 

 not only removed, but the level of the underground waters in the 

 sedimentary strata is so lowered that the hydrostatic pressure is no 

 longer equal to that exercised by the column of sea-water, so that, 

 instead of ;an outflow from the land, an inflow from the sea 

 necessarily takes place through the same channels or strata, and 

 thus, taking the place of the displaced fresh-water, finds it way to 

 the volcanic ducts. It is only Avhen from the exhaustion of the 

 fresh-water sources and the impeded access of sea-water, whether 

 owing to the resistance of the strata or to a decreasing hydrostatic 

 pressure, that the lava flows quietly and unaccompanied by the 

 violent explosions which mark the commencement of an eruption. 

 If, on the other hand, the sea-water gains access more freely through 

 the more porous volcanic materials, as in Stromboli and Kilauea, a 

 constant volcanic activity may be maintained. In ordinary cases, 

 however, where the inland waters, after the force of the eruption is 

 expended, regain the ascendant, they again exclude the sea-water, 

 and return to a state of equilibrium, which lasts until the strata are 

 again disturbed and fractured by a renewed eruption of lava. 



In conclusion, the author conceives that the first cause of volcanic 

 action is the welling up of the lava in consequence of pressure due 

 to slight contraction of a portion of the earth's crust. Secondly, the 

 fluid lava coming into contact with water stored in the crevices of 

 the masses of lava and ashes forming the volcano, the water is at 

 once flashed into steam, giving rise to powerful detonations and 



DECADE 11. — VOL. IX. — NO. I. 3 



