AFTER-EFFECTS IN CRYSTALLIZED IGNEOUS ROCKS 237 



these border zones he has grouped more recently as synantetic minerals, 

 or those formed where two older minerals meet. These reactions of the 

 very closing stages of crystallization and dependent on its mineralizers, 

 or "juice/' Sederholm proposes to call deuteric 15 — that is, they are second 

 or subsequent in development to crystallization proper. The results are 

 far more widely spread than petrographers have generally realized, and 

 once the observer's eye is quickened to note them, they will be frequently 

 detected. In some of our petrographic laboratories one may now hear 

 deuteric from every one's tongue as soon as questions come up which 

 affect the interpretation of what we actually see in the thin sections. 



XENOLITHS, MORE OR LESS DIGESTED 



Before we pass to the effects on the wall-rocks by the emissions, I may 

 mention one or two others which come to pass within the consolidating 

 rock-mass itself. The wall-rocks make contributions to the intrusive in 

 the form of fragments caught up when the magma stopes its way up- 

 ward, as we say, adopting the miner's term. While these xenoliths, in 

 stages 'of greater or less reorganization, are very frequent phenomena in 

 many igneous masses and may or may not display evidence of corrosion, 

 recrystallization, and saturation, there are two or three varieties specially 

 worthy of remark. 



ORBICULAR GRANITES 



All students of the igneous rocks are familiar with the curious phases 

 of the intrusive varieties which are known as orbicular or spheroidal 

 granites, or diorites, or gabbros, as the case may be. Stockholm is, per- 

 haps, the most accessible locality, but others are well known in Finland, 

 Ireland, Corsica, Sardinia, and elsewhere in Europe. In North America 

 boulders of excellent develo23ment have been discovered in Rhode Island 16 

 and in the lower peninsula of Michigan. 17 During the International 

 Geological Congress in Stockholm, in 1910, some of the delegates were 

 taken on an excursion to a granite quarry in the city, where the orbicular 

 granite was exposed. Discussions on the spot led to the conclusion that 

 the orbicular structures could be best explained by the digestion of frag- 

 ments from some wall-rock. Centers of abnormal composition were 

 thereby established around which, after the absorption of the inclusions, 

 recrystallization into the spheroids resulted. Our Michigan case gives 

 evidence of associated processes allied to the formation of pegmatites. 

 In any eveni?, the peculiar and striking structures are difficult to explain 

 without the aid of deuteric agencies. 



