132 Scientific Proceedings, Royal Dublin Society. 



or frothy portion, and the still continuous fluid mass beneath. If, 

 from the want of supply from below, this does not rapidly rise 

 and issue as a lava, it may go on gradually giving up its dis- 

 solved water in that state, and by loss of heat may solidify and 

 prevent, at any rate for some time, eruption at that particular 

 volcanic vent. In the types of tranquil activity, either strom- 

 bolian or in the case of occasional outpours of lava, 1 we have three 

 main agencies at work. In the first place, the aqueous matter 

 contained in the magma will be dispersed, proportionally lowering 

 the temperature. At the same time, the magma may be absorbing 

 fresh water, raising it to its own temperature, and eventually con- 

 verting it into vapour, which continues to escape at the expense 

 of its own heat. Last, and probably least important, would be 

 the conduction and convexion near the earth's surface by the 

 subterranean circulation of water. 



The Presence of Volatile Matter in Modifying the Structure and 

 Composition of Igneous Rocks. — As has already been intimated, 

 those grand explosive eruptions that burst forth, after long in- 

 tervals of complete tranquillity are characterized by an essential 

 ejectamenta of vesicular structure and fragmentary state. On 

 the other hand, chronic activity, even when it increases to the 

 stage of paroxysmal outbursts, is equally well marked by the 

 outflow of a continuous mass of igneous magma, or what is 

 generally known as lava. The vesiculur rock masses, or scoria, 

 that cover lava streams are, both in origin and structure, widely, 

 though not completely, different from the pumiceous products of 

 the first kind of eruption. These assertions hold true almost 

 without exception in the case of basic rocks, and are exceedingly 

 common even amongst the most acid ones. Most of the illustra- 

 tions that will be brought forward have been chosen from among 

 basic rocks, since, so to speak, crisis between a vitreous, fine, 

 vesicular, and fragmentary state on the one hand, and a crystal- 

 line compact continuous mass on the other, is easily attained, and 

 is well defined. 



1 The paroxysmal eruptions of Scrope, Volcanoes, 1828 and 1862. 



