714 A TREATISE ON METAMORPHISM. 



develop in a schist adjacent to an intrusive, when the continuations of the 

 schist formations remote from the intrusive contain little or no feldspar. 

 Such are the facts about the Black Hills batholiths (see p. 724) and at 

 manv places about the New-England granite batholiths. 



Another excellent illustration is furnished by the Bib Hill quartzites of 

 central Wisconsin, adjacent to a great intrusive batholith of augite-syenite. 

 The center of the quartzite mass contains no feldspar. As the masses of 

 syenite are approached feldspar. and hornblende appear in the quartzite. 

 These minerals increase in amount as the syenite is approached, and close 

 to that rock the quartzite is so thoroughly impregnated with feldspar which 

 has developed in and between the grains as to make many of the hand 

 specimens of the rock difficult to discriminate from a granite. 



There is much difference between the zones of anamorphism and kata- 

 morphism in the extent to which interchange of material takes place between 

 the injected and the injecting rocks. It has been seen that in the belt of 

 cementation, in consequence of the porosity of that zone, the material of 

 the magma, both by direct injection and by transmission through water, 

 may profoundly affect the average chemical composition of the intruded 

 rock for great distances from the intrusive mass. While these chang-es do 

 not take place on an extensive scale to any such distance from the intrusive 

 in the zone of anamorphism, we must not go too far in minimizing the 

 importance of the exomorphic effect of intrusive rocks in this zone. The 

 short distance to which magma can furnish material to the injected rock is 

 compensated to a large extent by the great scale of the intrusions. As has 

 been explained, there are in the zone of anamorphism innumerable batho- 

 liths, bosses, and stocks, and in temporary fractures intimate intrusions for 

 extensive areas. Hence the conditions favorable for endomorphic and 

 exomorphic effects without transportation of material to a great distance 

 from an intrusive are widespread and important. The quantity of solutions 

 is small, and they make their way very slowly through the subcapillary 

 openings. The consequence is that, where there is a great simple mass 

 of intrusive with few offshoots, the chemical compositions of the two rocks 

 are affected for only a short distance from each other; but where the 

 intrusive and intruded rocks are intimately mingled there may be a pro- 

 found modification in the chemical composition of both rocks over an 

 extensive area, each approaching the chemical composition of the other. 

 This general statement requires some amplification. 



