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not found it. The position of the arc is said to be determined by the 

 frontal slope of the mud lense built outward along the continental shelf. 

 This soft yielding mass is apparently supposed to behave as the stiff 

 rod, bent in the middle, which the author cites for comparison. The 

 arcs are believed to have been formed in succession from the old coign 

 outward. 



The theory is said to explain the sequence of lavas, in such regions as 

 the American Cordillera, from andesitic to rhyolitic and basaltic, and 

 also the facts of distribution of lavas in comagmatic regions. The first 

 fusion is supposed to affect the "average" shale and thus form an 

 andesitic magma. Later the fusion spreads to layers of siliceous shale 

 on the one hand and calcareous shale on the other, and from these are 

 derived the constitutents that change the original magma to siliceous or 

 basaltic composition. This is considered typical of the regions of active 

 folding. 



On the other hand the regions where the author believes faulting to 

 be dominant, such as the Atlantic basin, are characterized by basaltic 

 and alkaline lavas. The prevalence of these is explained by supposing 

 that, in the process of faulting, a massive competent layer of limestone 

 generally tends to separate from an underlying layer of calcareous shale. 

 This, being relieved of pressure, liquifies and forms a basic magma. 



In his concluding chapter, the author comments upon the tendency 

 of most scientific men to sanctify certain theories, and their tardiness in 

 recognizing the consequences of the passing of a discarded theory. 



Like previous works by the same author, the book is interesting and 

 suggestive, and it also reveals the same defects of loose reasoning, 

 inaccurate understanding of field facts and earlier writers, and hasty 

 conclusions. 



Eliot Blackwelder 



