DALY. — COMPOSITIONS OF IGNEOUS-KOCK TYPES. 



237 



from the earliest recorded time. There is no reason to doubt that the 

 diorite or the nephelite syenite of the pre-Cambrian periods have 

 generally owed their origin to the same physico-chemical reactions as 

 those responsible for the Mesozoic or Tertiary diorite or nephelite 

 syenite. If this be true, the world-averages for the different principal 

 types should be so many tests of theoretical conclusions as to the causes 

 of the differentiation of those types. The question as to the derivation 

 of augite andesite from basalt through fractional crystallization has 

 been thus tested, with, so far as this test goes, an affirmative answer. 4 

 Sometimes the averages themselves suggest lines of thought. For 

 example, the average granite analysis (calculated water-free ; 236 analy- 

 ses) is close to the average of four analyses of the glassy base of 

 augite andesite (calculated as water-free). The comparison may be 

 made from the following table : 



The exact meaning of the correspondence between the two averages 

 may not be discussed here ; but it does suggest an explanation of the 



* Journal of Geology, 16, 401 (1908). 



