CLASSIFICATION OF METAMORPHIC PROCESSES 409 



other areas, where the same formations were split hy many granitic 

 sills, the invaded rocks have quite different habit. The grain is charac- 

 teristically much coarser. The mineralogical composition is somewhat 

 nnlike that observed in the sill-free parts of the terrane. Nevertheless, 

 the invaded rocks are usually schistose in high degree; the planes of 

 schistosity are here, also, sensibly parallel to bedding planes, and the 

 directing influence of overlying load is as clear as elsewhere. The sedi- 

 ments appear to have been recrystallized by pure load-metamorphism 

 ))efore the epoch or epochs of sill injection. If so, this is another instance 

 of superposed metamorphisms. However, the evidence for a combination 

 of contact and load influences in the later metamorphism of the sill- 

 charged strata is clear. In fact, load metamorphism continued after the 

 freezing of the sills, for most of these are now orthogneisses, with schis- 

 tosity planes parallel to the sill contacts and to the planes of bedding in 

 sediments and volcanics. An influence which so fully controlled the re- 

 crystallization of a comparatively stable assemblage of minerals, like 

 granite, could not fail to direct the recrystallization of the strata alter- 

 nating with the sills. 



The writer suspects that load-contact metamorphism is largely respon- 

 sible for the development of Precambrian gneiss-schist complexes in gen- 

 eral. The sanction for the new term and for its definition depends in 

 part on the strength of the reasoning by which belief in pure load-meta- 

 morphism has been won. Using both principles, or at any rate keeping 

 an open mind on the question of their validity, one is better equipped for 

 an attack on the problem of the crystalline schists. 



PROPOSED CLASSIFICATION 



The following table gives the suggested division of metamorphic proc- 

 esses, each name bearing the preferred definition: 



A. Regioxal metamorphism (not caused by eruptive bodies). 



I. ^^tntic metamorphism (orogeuic movement not a causal condition). 



1. Stato-hydral metamorphism or hydrometamorphism (low 



temperature). 



2. Stato-thcrmal metamorphism or load motamoi-phism (high 



temperature) . 

 IT. Dj/Hdutic metamorphism (erogenic movement a causal condition). 



1. Dynnmo-hydral metamorphism or slaty (?) metamorphism 



(low temperature). 



2. Dynamo-thermal metamorphism or frietioa (?) metamor- 



phism (high temperature). 

 III. Dynamo-static metamorphism (load metamorphism in rooks lying 

 beneath overthrust masses). 



