TRANSACTIONS OP SECTION C. 611 



Mr. Poulett Scrope/ who consiilered that ' the rise of hxva in a volcanic vent is 

 occasioned by the expansion of volumes of high-pressure steam, generated in a 

 mass of liquefied and lieated matter within or beneath the eruptive orifice.' Ac- 

 cording to his view, the expulsion of the lava is effected solely by high-pressure 

 steam generated in the volcanic foci, hut no explanation is given how the water is 

 introduced. 



The objections to this hypothesis are — 1st. That during the most powerful 

 explosions, i.e. when the discharge of steam is at its maximum, the escape of lava 

 is frequently at its minimum, and vice versa. 



2ndly. That streams of lava often flow with little disengagement of steam, and 

 are generally greatest after the force of the first violent explosions is expended. 



3rdly. That it is not a mere boiling over, in which case, after the escape of the 

 active agent— the water — and the expulsion of such portion of the obstructing 

 medium, the lava, as became entangled with it, the remaining lava would subside 

 in the vent to a depth corresponding to the quantity of lava ejected; but the level 

 of the lava, ca'teris paribus, remains the same during successive eruptions. Of the 

 important part played by water in volcanic eruptions there can be no doubt, but 

 instead of considering it as the primarj', the author views it as a secondary cause 

 in volcanic eruptions. 



All observers agree in describing ordinary volcanic eruptions as generally ac- 

 companied or preceded by shocks or earthquakes of a minor or local character, to 

 which succeed paroxysmal explosions, during which vast quantities of stones, 

 scorite, and ashes, together with volumes of steam^ are projected from the crater. 

 The violence of the explosions gradually decreases and they then cease altogether. 

 The flow of lava, on the other hand, which commences sooner or later after the 

 first explosions, is continued and prolonged independently. Ultimately the vol- 

 cano returns to a state of repose, which may last a few months or many years. 



Adopting the theory of an original igneous (but now in greater part solid) nucleus 

 and of a thin crust, the author considers a certain fluidity of the former, or of a 

 portion thereof, and mobility of the latter, to be proved by the facts of the case. 

 The one and the other feebly represent conditions of which the phenomena of the 

 rocks afford clearer and stronger evidence as we go back in geological time. It is 

 estimated that a small quantity of central heat still reaches the surface and is 

 lost by radiation into space. It is evident also that even the escape of liquid 

 lava and steam from volcanoes, and of hot springs from these and other sources, 

 must bring, in however small a quantity, a certain increment of heat from the in- 

 terior to the surface, where it is lost. This must lead to some very slight con- 

 traction of the nucleus, and of re-adjustment of the external crust, in consequence 

 of which the fused matter of the interior from time to time tends to be forced 

 outwards. So far the author agrees with many other geologists. The additional 

 hypothesis which he now suggests, he has, however, been mainly led to form by 

 his researches on underground waters, and may be stated generally as follows : — 



A portion of the rain falling on the surface not only of permeable and fissured 

 sedimentary strata, but also of fissured and creviced crystalline and other rocks, 

 passes below ground, and is there transmitted as far down as the permeable rocks 

 range, or as the fissures in the recks extend, unless some counteracting causes 

 intervene. Those causes are the occurrence of impermeable strata, faults, and 

 heat. The former two are irregular, the latter one is constant. The increase of 

 temperature with depth being 1° Fahr. for every 50 to 60 ft., the boiling-point of 

 water would be reached at a depth of about 10,000 ft., but, owing to the pressure 

 of the superincumbent rocks, it has been estimated that water will retain its 

 liquidity imd continue to circulate freely to far greater depths, but a point will 

 be reached where the tension of the heated water will equilibrate the pressure. 



Very little is known of the substrata of volcanoes. Etna and Hecla apparently 

 stand on permeable Tertiary strata, Vesuvius on Tertiary and Cretaceous strata, 

 while in South America some of the volcanoes are seemingly situated amongst 



' For reasons that will be app.nrent, the author does not discuss the chemical 

 theory of Davy or the ingenious theruio<lvnnmical theory of Mallet. 



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