IMPERFECT CHARACTER OF TUE PREMISES. 139 



quartz-propylites as contributing their quota of support to the theory, we 

 may still hold that they are not necessarily in conflict with it. 



There is another conceivable mode in which the law here propounded 

 theoretically may be modified in a manner which would yield results dif- 

 fering from the standard sequence to which it has been applied and give a 

 somewhat different but still a definite succession. It might be affected by 

 the depth at which the seat of volcanic activity is located, and also by the 

 value of the mean density of the overlying rocks. Assuming our theory 

 to be correct, let us call the depth at which Richthofen's succession becomes 

 the normal one, unity. Suppose the depth to be considerably greater than 

 unity, the melting temperature of the acid rocks would then be greater on 

 account of the increased pressure. Recurring to the graphic diagram, the 

 effect of this modification would be to transfer the intersection of the fusion 

 and density curves to the left or toward the basic end of the scale, and 

 rocks more basic than propylite would be first erupted and the succession 

 would be more or less modified. The nature of the modification will 

 readily appear by treating the modified diagram in the same manner as has 

 been employed already. Or suppose the depth of eruptive activity to be 

 less than the assumed unity : the intersection of the two curves would 

 be transferred to the right and an inverse series of modifications would 

 result. On the assumption that the secular cooling of the earth is gradu- 

 ally sinking the seat of volcanicity to lower horizons, it would follow that 

 a corresponding modification is secularly proceeding in the normal order 

 of succession in volcanic eruptions. 



This theory has one important element of weakness which it is neces- 

 sary to point out. The assumption that the proximate cause of volcanic 

 activity is an increase of temperature is to a great extent an arbitrary one. 

 Conclusive proof of it does not seem to be obtainable at present. There 

 are numerous indications of it, many facts which seem to point to it; yet 

 that strong, convincing evidence which can entitle such a proposition to 

 absolute confidence is wanting. Hence the theory should be called rather 

 a trial hypothesis, in which there is an important premise which remains to 

 be proven. It is a frequent resort, however, in all sciences to adopt such 

 premises provisionally, and they gain strength or the contrary in proportion 



