552 N. L. BOW EN 



in discussing the effects of basic magma on aluminous sediments. 

 This affords an opportunity of discussing the behavior of an actual 

 example of aluminous sediment. The ordinary Manhattan schist 

 is made up principally of the minerals quartz, biotite, muscovite, 

 orthoclase and plagioclase. These are all minerals of ordinary 

 igneous rocks, particularly of more "acid" types, and correspond to 

 a rather low temperature equilibrium. A glance at the analysis 

 shows, however, that the composition is far from that of an ordinary 

 igneous rock, which means that the minerals are of somewhat differ- 

 ent composition and are present in different proportions. Now we 

 have found in our discussion of the reaction of any saturated magma 

 upon igneous inclusions that if the inclusions belong to a later stage 

 in the reaction series they may become a part of the liquid by caus- 

 ing the precipitation of the phases with which the liquid is saturated. 

 Average Manhattan schist, since it consists of the minerals of an 

 acidic igneous rock, may be regarded as consisting in part of material 

 belonging to a later stage in the reaction series than basaltic magma, 

 but since it does not correspond exactly with any such igneous mass 

 it must be regarded as having a certain amount of surplus material 

 in addition. If we imagine saturated basaltic magma reacting with 

 inclusions or wall rock of schist we may expect the action to be 

 selective. Such substances as may become a part of the liquid 

 would be removed from inclusions or wall rock with corresponding 

 enrichment in what has been called surplus material. The sub- 

 stances removed would be principally silica, alumina, alkalis and 

 to a minor extent other oxides, all in the proportions in which they 

 enter into some "acid" igneous rock. Our knowledge of the exact 

 proportions may be thus indefinite and yet sufficient for a general 

 solution of the problem. Comparison of the analysis of the Man- 

 hattan schist with those of acid igneous rocks gives us a good con- 

 ception of what the surplus material will be. It will plainly be 

 rich in alumina and iron. A certain stage of the reaction between 

 magma and inclusions or wall rock should exhibit a mass rich in 

 these oxides. This stage is abundantly represented in both wall 

 rock and inclusions by richness in sillimanite, staurolite and other 

 aluminous and ferrous minerals. Chemically it is shown by analysis 

 II in Table III which represents wall rock at the margin of the 

 intrusive. 



