134 Scientific Proceedings, Royal Dublin Society. 



From, this Table we learn that a series of intermissions take 

 place in the activity of a volcano ; that following these inter- 

 missions we have the production of fragmentary pumiceous ejecta- 

 menta, which, by gradual stages, pass through a pumiceous scoria 

 stage to that of true lavas. The striking resemblance between 

 the phenomena of the three first-named volcanoes up to a certain 

 stage cannot but strike the observer as very remarkable, and opens 

 9, wide field of investigation, for it is quite certain that similar 

 stages in each of these volcanoes were not contemporaneous. 



The igneous magma, whilst confined below, may have under- 

 gone partial crystallization before it reaches the surface. In that 

 case, however rapid the ejection and cooling be, the ejectamenta 

 will always contain those formed materials, as seems to have 

 been the case with sanidine, amphibole, and, perhaps, other mine- 

 rals in all; the pumices of Vesuvius, Mt. Yultura, Eoccamonfina, 

 the Campi Phlegrei, Ischia, &c. In case of the magma containing 

 a large amount of diffused water, the sudden and rapid conversion 

 of this into the gaseous state will immediately result in the con- 

 version of the magma into a spongy mass, splitting it up into 

 fragments of various sizes by the partial escape of the gaseous 

 contents, and rapidly absorbing an enormous amount of heat. In 

 consequence, the mass solidifies before time is given for the con- 

 version of the vitreous into the microcrystalline or crystalline 

 state, or, at the most, only allows such to take place imperfectly. 

 As a result of the rapid solidification, many of the bubbles of gas 

 are unable to escape from the mass, except where near the surface. 

 We must remember that the change of pressure is not only from 

 that of the original magma to that of the atmosphere at the 

 earth's surface, but the low pressure reached, by the ejectamenta, 

 many thousands of feet, or even some miles upwards, to which 

 height the materials are projected ; and even if that were not 

 sufficient, the rapid cooling by contact with the cold air in falling 

 would complete the refrigeration. That such is really the case 

 we have certain proof of in the preservation of organic and easily 

 fusible substances of Pompeii. The actual physical structure and 

 mineral composition of a pumice will depend on — 



(a) Composition of the original magma. 



(b) Pre-eruptive temperature of same. 



