I40 NATURAL SCIENCE. Feb., 1894. 



and of higher tension, it will burst forth, forming the final phase of the 

 volcano. 



What may be the composition of the fundamental paste is still 

 an open question, but I believe that it differs in different regions, so 

 that in one volcanic province potash may be a dominant alkali, and in 

 another soda, and so on. So far as my own individual opinion goes, 

 I believe it was a rather acid rock, corresponding to a trachyte, 

 and that either extremes have been caused by chemical alteration. 



In fine, I do not deny that the modern hypotheses may each 

 in part explain the cause of differentiations of pastes, but I do maintain 

 that neither the " focal diffusion," " segregation," nor '* sedimentation " 

 processes are sufficient by themselves to do so. 



H. J, Johnston-Lavis. 



