CLASSIFICATION OF METAMORPHIC PROCESSES 403 



for one to assume that load has been a main factor in the metamorphism 

 of many of the plateau terranes listed. 



Again, for some schistose formations which have been greatly dislocated, 

 there is good evidence that the principal metamorphism was accomplished 

 before the main foldings or faultings. An nnnsiially vivid instance is 

 found in the British Columbia Shuswap terrane. On its gneisses and 

 schists rest, in nearly perfect structural conformity, the extremely thick 

 Beltian sediments. These, nearly or quite conformably, pass upward into 

 the Cambrian series. All three rock groups have been upturned in post- 

 Cambrian time. Some of the Cambrian beds, much of the Beltian series, 

 and almost all of the Shuswa]) sediments and eruptives had l)ee]i thor- 

 oughly recrystallized before the upturning. That deformation caused 

 new, quite local dynamic metamorphism, but left the original schistosity 

 largely unchanged. The post-Cambrian orogeny seems to have had noth- 

 ing to do with the principal schistosity. 



A similar relation prevails in the Erzgebirge, Vogtland, the Fichtelge- 

 birge, and East Thuringia, where the "Arcbean" gneisses and schists pass 

 up, concordantly, into thick phyllites, and these up into fossiliferous (Jam- 

 brian sediments (Credner, 1897, page 396; Lepsius, 1903, page 108; 

 Rosen])usch, 1910, page 577). 



Lory (1888, page 87) described it in the Monte Eosa district of the 

 Alps. 



A third reason for crediting the great etRciency of load metamorphism 

 is the exceedingly common parallelism between foliation or schistosity and 

 the stratification. This fact is abundantly illustrated in the (*anadian 

 shield, in the Adirondacks of JSTew York State, and in the Precambrian 

 of the N'oi'th American Cordillera, Scotland, Scandinavia, Finland, etcet- 

 era. Lowl (1906, page 50) bas given a good statement of it in the fol- 

 lowing passage (translated) : ''Tlie great majority of the crystalline schists 

 are foliated, not across the bedding, but parallel to it. Their parallel 

 texture must have been developed when the rocks lay undisturbed, and 

 thus only because of the downward pressure of the overlying rocks, exactly 

 as in the case of shale and iiLost clay-slates, among which, indeed, trans- 

 verse cleavage is not the rule, but the exception. It is not merely a case 

 of the condensation of the buried rock by the dead weight of its cover. 

 The load also causes foliation. Its effect is not hydrostatic, but, even if 

 there be pressure on all sides, the pressure in the vertical direction is the 

 strongest. Lateral thrust may develop still greater inequality of pressure, 

 especially at small (lc])tli : yet aii essential difference between the effects 

 of load and latcial thrust is not to l)e assumed." 



Parallelism of schistosity and bedding, to the degree observed in the 



XX[X r.i I.I,. Gk,.!,. S..r. Am.. A'oi. 28, tniO 



