422 



N. L. BOWEN 



squeezed upward by lateral compression of part of the gabbro 

 mass. Grout, in offering objections to crystallization-differentiation 

 as an explanation of the red rock and in discussing in that con- 

 nection the composition of the liquid, states that ''before it reaches 

 the composition of such a granophyre as the red rock of Duluth a 

 large amount of feldspar must have crystallized from a magma too 

 acid to yield basic labradorite. "' On another page, however, he 

 describes the red rock as amounting to "less than 300 feet of acid 

 andesine. " Now the liquid that is in equilibrium with AbiAua 

 crystals is precisely of the composition of acid andesine, to be more 



Fig. 5. — Outline map of Duluth gabbro (after Grout) 



specific AbjAui, so that Grout's objections fall when examined in 

 the light of his own detailed descriptions. If the red rock repre- 

 sented an immiscible liquid it is rather remarkable that at the time 

 at which it had separated it should have a composition appropriate 

 for this final plagioclase composition. 



In connection with this particular association, namely that of 

 granophyre and gabbro, a rather definite suggestion as to conditions 

 of possible immiscibility has been offered. It has been suggested 

 that on account of immiscibility between water and silicates the 

 magma separates into two liquids, the one water-rich and the other 



^ "A Type of Igneous Differentiation," Jour. Gcol., XXVI (1918), 645. 



