I04 5E0CEEDINGS OF THE GEOLOGICAL SOCIETY. 



in the same volcano at different times, and between the eruptions 

 of different volcanoes, is a subject deserving the most attentive con- 

 sideration. The violence of a volcanic outburst does not bear any 

 relation to the quantity of material ejected. The union of water 

 with lavas may be compared with the solution of a gas in water ; 

 but there is reason to believe that in their deep-seated sources lavas 

 contain little or no water. If igneous matter be extruded through 

 dry strata, the eruption might take place without explosive manifes- 

 tations. But if igneous matter be extruded through water-bearing 

 beds, a kind of dialysis would take place between the igneous and 

 aqueous masses. The amount of water dissolved in the magma will 

 be proportional to the length of time it is in contact with the aqui- 

 ferous strata, the pressure, and the temperature of the fluid rock ; 

 the violence of the eruption will depend upon the amount of igneous 

 matter, the quantity of water dissolved in it, and its temperature 

 when it reaches the surface. 



The intrusion of igneous matter into dry or nearly dry strata, 

 the temperature and bulk of the magma will determine whether a 

 sahlband is formed, or formed and re-fused, whether the rock cools 

 quickly, forming a fine-grained structure, or very slowly, in which 

 the result would be a coarse granite or syenite, according to the 

 composition of the magma. 



The author showed how the cleavage- and stratification-planes ef 

 rocks are suitable to the retention of subterranean heat. Intrusion 

 of igneous matter into water-bearing strata was then studied, and it 

 was shown that a process of dialysis goes on between the colloidal 

 magma and the water in the porous strata, resulting in many inter- 

 esting phenomena. The loss of heat in the magma from the ab- 

 sorption of water will be little, and only that necessary to raise the 

 water to its own temperature. It was shown how this absorption of 

 water will result in the rupturing of the fissure towards the surface ; 

 and the mechanism of a certain group of earthquakes was investi- 

 gated. The occurrence of vesicular structure in dykes was discussed ; 

 the mode of formation and probable process of re-obliteration is 

 dependent upon the variation in pressure, temperature, &c. resulting 

 from the enlargement of the fissure. It was then shown that the 

 cooling of a dyke-mass wiU depend on the following conditions : — 



(a) Loss of heat from conduction away by the surrounding 



rocks. 

 (h) Eaising the acquired water to the mean temperature of 



the fluid of the fused silicates in which it is dissolved. 



(c) Heat-loss in consequence of expansion during extension 



of fissure. 



(d) Gradual escape of water in the form of steam or vapour 



through fissures, so supplying fumaroles. 



(e) Convection-currents of water forming geysers or thermo- 



mineral springs. 



The author combatted the theory that the simple contact of the 

 molten rock with water-bearing strata is the cause of an eruption. 



