242 POPULAR SCIENCE MONTHLY. 



the earth's surface, molten material may be forced througli it and 

 a volcanic eruption initiated. In brief, variations in one process 

 may lead to tlie formation of dikes, sheets, laccolites, and other 

 forms of intrusion, and to volcanic eruptions. Possibly all these 

 results might follow from the opening of a single fissure. 



In all the instances enumerated the magma may be the same, 

 so also the source of the heat and the origin of the pressure which 

 forces the molten rock into the earth's crust, or causes it to be dis- 

 charged at the surface. 



Returning to the hypothesis that steam is the mainspring of 

 volcanic phenomena, it will be conceded, I think, by most observ- 

 ers that steam contained in a magma can not be called upon to 

 account for its rise in a deeply seated fissure so as to form a dike, 

 or, when the conditions are varied, give origin to laccolites, etc. 

 Not only are the rocks composing such intrusions, the densest of 

 igneous rocks, but they are without steam cavities. Besides the 

 filling of a fissure with plastic or fluid rock, and still more strik- 

 ingly in the production of other varieties of intrusions, a bulk of 

 matter, measured in some instances by cubic miles, is forced in 

 among the solid rocks of the earth's crust. There is thus a bodily 

 transfer of matter, frequently for long distances, from one place 

 to another deep within the earth's crust and against an enormous 

 pressure. All these facts are adverse to the conception that bodies 

 of liquid or plastic rock are moved by the expansive force of the 

 steam contained in them. The energy expended in producing igne- 

 ous intrusions is in numerous instances so far in excess of that 

 manifest in any explosive volcanic eruption that has been record- 

 ed not excepting Cosequina, Sumbawa, or Krakatoa, but rather 

 combining them all and more in one that it becomes of a differ- 

 ent order of magnitude, and a different origin is to be suspected. 



In the case of subterranean injections, it is evident that the 

 source of the lieat which renders the rocks plastic, and the source 

 of the pressure which forces the plastic material into fissures, etc., 

 are distinct and should be separately considered. 



The heat manifest in both subterranean and surface igneous 

 phenomena, as is well known, has been variously accounted for, 

 but I do not wish to consider this problem at present. The con- 

 sensus of opinion, however, seems to be that the heat referred to 

 is mainly and essentially the internal heat of the earth i. e., the 

 residual heat of a cooling globe. It is conceded also that the mat- 

 ter composing the earth at a depth of a few miles below the sur- 

 face is so highly heated that it would become plastic or even highly 

 fluid if the pressure under which it exists were removed. The best 

 conception we can frame of the general physical condition of the 

 earth is, that it consists of a more or less spherical mass, which is 

 highly heated and in a potentially plastic condition within, and 



