REVIEWS 743 



in a magma-basin, it is necessary to employ observations upon series 

 of genetically connected eruptions, of a definitely bounded region, 

 and belonging to a single eruptive period. The work of Iddings, 

 Teall, Dakyns, and Wadsworth has shown that the norftial series is 

 basic — less basic — acid. This agrees with the explanation of differ- 

 entiation through a diffusion toward the cooling surface of a magma- 

 basin, which is regulated by the ordinary sequence of crystallization. 

 According to Brogger the effusive rocks are in great part the products 

 of a secondary differentiation from plutonic magmas ; and, in gen- 

 eral, the further away the magma-masses are from the original mother- 

 magma, the more they have been subjected to secondary differentiation 

 and the less regularity is observable in their eruptive sequence. 



F. L. Ransome. 



