334 ORIGIN OP ERUPTIVE AND PRIMARY ROCKS. 



But this does not prove that a fused silicate, the constituents of 

 which are already in chemical combination with each other, ex- 

 periences a dimunition of density or increase of volume in cool- 

 ing and crystallizing. The only instance of the cooling of 

 a fused silicate, which has been made the subject of observation 

 so far as regards density is the formation of Reaumur's porcelain 

 from glass, but this goes rather to prove the oppositeof Von Walter- 

 shausen's theory. Many kinds of glass, after exposure for several 

 hours to a heat at which they become soft, pass into a condition 

 resembling porcelain, become opaque, doubtless from the separa- 

 tion of fine particles, whose composition differs from the mass. 

 The resulting "Reaumur's porcelain" is specifically heavier than 

 the glass from which it is prepared. Moreover, this substance 

 when again fused and rapidly cooled yields an enamel the specific 

 gravity of which is to that of the substance before fusion as 2,625 

 is to 2,801.* From this it would appear that instead of an increase 

 a diminution of volume takes places in the slow cooling or crys- 

 tallization of fused silicates. 



If we reject both the hypotheses just mentioned, the only expla- 

 nation left, whereby the ascent of the lava column may be account- 

 ed for is that which is regarded as the cause of the more wide- 

 spread earthquakes/viz. the fluctuations of the surface of the fluid 

 interior of the earth.f While those earthquakes which occur 

 simultaneously with volcanic eruptions and in volcanic districts 

 may be considered as a consequence of the lava rising in the 

 volcano, the same can scarcely be said of those earthquakes which 

 occur in the midst of continents far distant from any volcanic 

 region. According to Naumann, the most probable cause of these 

 "plutonic" earthquakes is "a fluctuation of the surface of the 

 " fluid kernel of the earth commencing from a line or a point, 

 " and progressing according to the laws of the motion of waves." 

 The cause of such fluctuations he leaves undecided, but in com- 

 menting npon vonHoff's, Merian's and Perrey's investigations as to 

 the greater frequency of earthquakes in certain seasons of the year 

 he propounds a question, the consideration of which would seem to 

 yield the most important results. The investigations referred to 

 established the fact that in the northern hemesphers in winter earth- 

 quakes are of greater freueney than during any other seasons. Von 

 Hoff found that of the 115 earthquakes which, during the 10 years. 



* Gmelinlll, 385. 

 \ Naumaan : Lehrbuch, I, 291. 



