30 Notices of Memoirs — Causes of Volcanic Action. 



lY. — Some Observations on the Causes of Volcanic Action. 

 By J. Prestwioh, M.A., F.E.S., etc., Professor of Geology in the 

 University of Oxford, 



(British Association Eeports : Section C. York Meeting.) 



CONSIDERABLE difference of opinion still exists as to the cause 

 of volcanic action. The hypothesis, however, generally accepted 

 in this country is that of the late Mr. Poulett Scrope, who considered 

 that " the rise of lava in a volcanic vent is occasioned by the 

 expansion of volumes of high-pressure steam, generated in a mass 

 of liquefied and heated matter within or beneath the eruptive ori- 

 fice." According to his view, the expulsion of the lava is effected 

 solely by high-pressure steam generated at great depths, but at 

 what depths is not mentioned, nor is it explained how the water 

 is introduced, whether from the surface or whether from water in 

 original combination with the basic magma. 



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

 powerful explosions, i.e. when the discharge of steam is at its maxi- 

 mum, the escape of lava is frequently at its minimum. 



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

 of steam, and are generally greatest after the force of the first violent 

 explosion is expended. 



Srdly. 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, ccsteris 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 primary, the 

 author views it as a secondary cause in volcanic eruptions. 



All observers agree in describing ordinary volcanic eruptions as 

 generally accompanied or preceded by shocks, or earthquakes, of a 

 minor or local character, to which succeed paroxysmal explosions, 

 during which vast quantities of stones, scoriae, and ashes, together 

 with volumes of steam, are projected from the crater. The first 

 paroxysms are the most violent, and they gradually decrease and 

 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 volcano returns to a 

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



Adopting the theory of an original igneous nucleus, but without 

 going into the controverted question of the present state of that 

 nucleus and the outer crust, the author considers a certain fluidity 

 of the former, and mobility of the latter, or of a portion thereof, 

 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. 

 Although thermometrical experiments of the necessary accuracy 

 and length of time are yet wanting, it has been estimated that a 

 small quantity of central heat still reaches the surface and is lost 



