LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 89 



zone of contact metamorphism or in the hybrid rock already referred 

 to. Thus garnet is likely to be precipitated at some stage in the 

 crystallization of such a magma, also scapohte, sillimanite, and 

 even calcite when the magma is not very rich in silica, as in the case 

 of foyaitic magma. It may well be, also, that some meliUte rocks 

 are formed by the crystallization of a syntectic magma formed by 

 the solution of Umestone, as Daly has suggested.^ The precipitation 

 of such unusual minerals is, however, usually only an incidental 

 feature in the crystallization of the magma. The whole course of 

 crystallization will probably still be dominated by the feldspars 

 and the continual offsetting toward the more alkalic feldspars will 

 be maintained. Probably, then, the crystallization of such magmas 

 will tend to pass down through the same intermediate stages to the 

 granitic and into the alkalic stage with the occasional appearance 

 of some unusual mineral and a slightly increased or decreased 

 amount of precipitation of some of the normal minerals at various 

 stages. 



If the absorption of any considerable amount of limestone by 

 a magma can be admitted, it may be expected to have a very 

 unusual effect upon the magma, as Daly has suggested. The 

 taking of silica from feldspar molecules by the lime and the conse- 

 quent production of feldspathoid molecules might well be supposed 

 a reasonable possibility. Some alkahne rocks may, perhaps, be so 

 generated, but on an earlier page reasons have been presented for 

 believing that normally the alkahne rocks enter into an eruptive 

 sequence as the products of differentiation solely. 



SUMMARY AND CONCLUSION 



It is desirable now to summarize briefly the principal con- 

 clusions reached. Consideration of the factors Hmiting its scope 

 has led to the decision that assimilation is, relatively speaking, an 

 unimportant factor in the production of the diversity of igneous 

 rocks. For pahngenetic action in its still more extreme form, 

 involving the direct refusion of sedimentary terranes, no support 

 is found. Such action would often give rocks of mineral composi- 

 tion never found in an igneous series, rocks consisting mainly of 



^ R. A. Daly, Igneous Rocks and Their Origin, p. 436. 



