402 K. A. DALY METAMOKPJIISM A^'D JTS PHASES 



In spite of the clear aniioiinceiiieiits of the principle b}^ Dawson and 

 Milch, many geologists of the present day are still far from sympathetic 

 with the idea of load metamorphism. Very rarely is it even mentioned 

 in works on rock changes or on the crystalline schists. Of the few au- 

 thors who liave considered vertical stress in relation to the development 

 of schistosity, a nnmher like A. Geikie, Van Plise, and Pirsson express 

 donbt that there is any important positive relation between them. For 

 tliese reasons a sketch of the held facts, suggesting the reality and great 

 significance of load metamorphism, may not be without warrant. The 

 value of a classification of the phases of metamorphism really depends in 

 no small part on a wise decision in this matter. 



A strong, perhaps the strongest, argument for load metamorphism is 

 expressed in the foregoing quotations from DaAvson and Termier. Ex- 

 tensive areas in western Canada, eastern Canada, and the western Alps, 

 though underlain by typical crystalline schists, display no evidence of 

 ever having been greatly aifectecl by crustal deformation. Bedding is 

 l)erfectly preserved in the sedimentary members of the crystalline groups 

 of rocks. The di]3 is cliaracteristically low, even nearly horizontal, over 

 wide stretches. The structure is that of a plateau, a somewhat broken 

 plateau. 



Besides the examples in the Belt terrane and Shuswap terrane of the 

 American Cordillera, in Ontario, and in the Alps (compare Loiy, 1888, 

 page 87 if), many others are on record. In this list of regions are notable 

 tracts in Labrador (Low, 1895, page 199) : the Adirondacks (Cushing, 

 1914, page 30; W. J. Miller, 1914, page 59, and 191G, page 587) : Green- 

 land (A. Ileim, 1911, page 180) ; the Gfohl gneiss of the Lower- Austria 

 Waldviertel (Becke, 1910, page 617); the Erzgebirge (Lepsius, 1903, 

 pages 89, 99, 108); the Schwarzwald (Scliwenkel, 1912, pages 139 and 

 253); the Oban-Dalmally district of Scotland (Kynastoii, 1908, page 

 21) ; German East Africa (Schmidt, 1886, page 451, and Bornhardt, 

 1900, page 459) ; Rhodesia, Congo State, and Uganda (Mcnncll, 1913, 

 page 205). 



With a few exceptions, the authors mentioned do not consider in print 

 the cause of the ^^lateau structnre in the respective gneisses and schists. 

 The extreme advocates of dynamic metamorphism would find it in "roll- 

 ing-out," "overturning of folds/' "multiple thrusts," or intense horizontal 

 shearing. In the fields studied l\v the writer none of these explanations 

 can be admitted. As Dawson clearly saw, tbe only feasilde explanation 

 of the schistosity in the flat-lying Shuswap rocks is dead-weight stress 

 controlling their recrystallizatioii. Sup|)()itcd also by the opinions of 

 Milcli, Termier, Lory, Becke, ;iii,(l SchMcnkel, it is not altogether rash 



