FUSION AND ABSORPTION. 731 



adjacent to great batholiths show various stages of absorption. Frequently 

 the partly absorbed residual fragments are profoundly metamorphosed, 

 being perhaps completely recrystallized, and frequently greatly changed in 

 chemical composition from that of the parent rock. 



It is certain that the_ absorption of fragments must react upon the 

 chemical composition of the absorbing rock. In so far as rocks fused and 

 absorbed by a mag-ma differ from it in chemical composition, they must 

 change the chemical composition of that magma. Where the rock absorbed 

 is igneous, and therefore has the composition of igneous rocks, the 

 modified magma has a composition intermediate between two magmas. 

 Where the material has the composition of a sediment, this changes the 

 chemical composition of the magma toward that of the sediment. For 

 instance, if a considerable portion of limestone or dolomite be absorbed, 

 the rocks become very rich hi calcium or calcium and magnesium. If a 

 considerable portion of shale or slate derived from mudstone be absorbed, 

 the igneous rock would be likely to be deficient in the alkalies, especially 

 sodium; it might be deficient in the alkaline earths; and it would probably 

 be high in aluminum, thus becoming intermediate in chemical composition 

 between igneous rocks and the mudstone series. If a sediment intermediate 

 between the limestone series and the mudstone series, as, for instance, 

 calcareous shale or slate, were absorbed by magma, while there might not 

 be any deficiency in the alkaline earths, there would be a deficiency in the 

 alkalies, and probably the aluminum would be abnormally high. 



If a rock were fused by contact with a magma without absorption by 

 it or mingling with it, it would, of course, have approximately the compo- 

 sition of the original solid rock, and therefore that of a sedimentary, igneous, 

 or metamorphic rock, as the case might be. Doubtless the compositions of 

 such rocks would be somewhat modified by the exomorphic effect of the 

 igneous rocks. (See pp. 713-714.) If any considerable mass of solidified 

 rock were fused as a result of contact with igneous rocks, it is natural to 

 suppose that for a zone of variable width the two would become mixed, and 

 thus there would be a gradation in chemical composition between the fused 

 rock and a normal magma. 



While it is beyond dispute that all of the above cases occur, petrog- 

 raphers have been inclined to believe that none has taken place on a great 

 scale. It may well be doubted whether the excess of heat in molten magma, 

 beyond that required to keep it liquid, is sufficient to perform the vast 



