1885.] 



Agency of Water in Volcanic Eruptions. 



253 



is about an inch in length, and a quarter of an inch across the instep, 

 from which it tapers elliptically to a pointed toe, while the heel at the 

 anterior extremity is abruptly truncated and broadly notched. 



The slipper is ornamented with eight principal spiny ridges, taking 

 a longitudinal direction, diverging from the toe and terminating 

 separately at the heel. But the central vertical ridge corresponding 

 with the front of the slipper is cut off where a square notch or fissure 

 in the instep receives the muscular attachment of the animal. The 

 whole scheme of arrangement will be better understood on inspecting 

 the accompanying figure, which is about three times the natural size. 



III. " 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., 

 Professor of Geology in the University of Oxford. Received 

 March 26, 1885. 



(Abstract.) 



That water plays an important part in volcanic eruptions is a well- 

 established fact, but there is a difference of opinion as to whether it 

 should be regarded as a primary or a secondary agent, and as to the 

 time, place, and mode of its intervention. The author gives the 

 opinions of Daubeny, Poulett-Scrope, and Mallet, and dismissing the 

 first and last as not meeting the views of geologists, proceeds to 

 examine the grounds of Scrope's hypothesis — the one generally 

 accepted in this country — which holds that the rise of lava in a 

 volcanic vent is occasioned by the expansion of volumes of high 

 pressure steam generated in the interior of a mass of liquefied and 

 heated mineral matter within or beneath the eruptive orifice, or that 

 volcanic eruptions are to be attributed to the escape of high pressure 

 steam existing in the interior of the earth. The way in which the 

 water is introduced and where, is not explained, but as the expulsion 

 of the lava is considered to be due to the force of the imprisoned 

 vapour, it is, of course, necessary that it should extend to the very 

 base of the volcanic foci, just as it is necessary that the powder must 

 be in the breech of the gun to effect the expulsion of the ball. 



The author then proceeds to state his objections to this hypothesis. 

 In the first place, he questions whether it is possible for water to 

 penetrate to a heated or molten magma underlying the solid crust. 

 The stratigraphical difficulties are not insurmountable, although it is 

 well known that the quantity of water within the depths actually 



