On the Agency of Water in Volcanic Eruption?, 



117 



" On the Agency of Water in Volcanic Eruptions ; with some 

 Observations on the Thickness of the Earth's Crust from a 

 Geological Point of View ; and on the Primary Cause of 

 Volcanic Action."* By Joseph Prestwich, F.R.S., F.G.S., 

 &c. Received March 26. Read April 16, 1885. 



[Plate L] 



PAGE 



§ 1. Introductory Observations — Current Hypotheses — The Yapour of Water 



considered as the Primary Cause of Volcanic Action 117 



§ 2. Objections to this Hypothesis 120 



§ 3. Influence of Volcanic Eruptions on Spring and Well Waters 133 



§ 4. The Hydro-geologieal and Statical Condition of the Underground Waters 



in and under a Volcanic Mountain 137 



§ 5. Condition of the Underground Waters during an Eruption 146 



§ 6. Thickness of the Earth's Crust from the Greological Standpoint 158 



§ 7. The Primary Cause of Volcanic Action 170 



§ 1. Introductory Observations — Current Hypotheses — The Vapour of 

 Water considered as the Primary Cause of Volcanic Action. 



The important part played by water in volcanic eruptions is a well 

 recognised and established fact, but there is great difference of opinion 

 among geologists as to whether water should be considered the 

 primary or secondary agent, and as to the mode, time, and place of its 

 intervention. The prevailing opinion in this country is that water is 

 the primary cause of volcanic activity. Whichever view may be 

 adopted, the subject is one which is so largely concerned with the 

 laws regulating the underground circulation of water, that the con- 

 sideration of the two questions must proceed pari passu. We shall 

 therefore have to consider somewhat fully the hydro-geological ques- 

 tions relating to the circulation and penetration of water, and in con- 

 nexion with this the contested question of the probable thickness of 

 the earth's crust from the geological standpoint. The objections to the 

 chemical theory of Davy, according to which, water finds its way to 

 the interior of the earth, and there, meeting with the metals of the 

 earths and alkalies, is decomposed with the evolution of intense heat, 

 steam, and gases, have been so often stated, that it is not necessary 

 here to refer to them further than to remark that the objections I shall 

 have to urge generally against the percolation or passage of water 

 to extreme depths will apply equally to this hypothesis also. 



The theory of volcanic action which has of late years been most 



* The general views expressed in this paper were laid before the Geological 

 Section of the British Association at the York Meeting in 1881. See Report of 

 Section C, p. 610. 



