382 II. A. DALY MI^TAAIOIUMIISM AND ITS PHASES 



Turning to J. D. Dana's Manual of Geology, one reads (1895, page 

 310) : "Under metamorphism might be included the chemical changes in 

 rocks and minerals that take place at ordinary temperature. But these 

 run down into the common results of decay and are more conveniently 

 kept separate." While thus excluding weathering, Dana thought that the 

 cementation of sediments should be regarded as a phase of metamorphism. 

 He distinguished local and regional metamorphism, the latter being (1) 

 incipient, (2) crystallinic, (3) paramorphic, (4) metachemic, or (5) 

 endo-crystallic. According to the source of the heat involved, he further 

 distinguished "statical" and "dynamical" metamorphism. Statical meta- 

 morphism is (page 440) "that dependent on heat of a statical source — 

 tlie earth's mass and the vapoi's about it." This kind characterized tlie 

 "Litliic Era" in the globe's history, the long period befoi'e the ocean con- 

 densed on tlie original crust. In this inatter Dana was evidently not a 

 uniformitarian. 



His Archeozoic a^on was (page 441) characterized by dynamical meta- 

 morphism, which is "dependent on heat from a dynamical source— that is, 

 ]ieat generated by movements in the thickening crust." On the other 

 iiand, lie notes (page 322) that "the earth's internal heat has always been 

 a contributor to the heat of the earth's crust, and much more so formerly 

 than now, and would, therefoi'e, have supplemented largely the heat gen- 

 erated by friction." From a passage on the same page of the IVfanual 

 one must infer that Dana regarded "dynamical metamorphism" and 

 "regional metamorphism" as rigorons synon3mis. 



In the last edition of his Textbook of Geology, A. Geikie (1903, page 

 424) defines metamorphism of rocks as "rearrangement of their constitu- 

 ent minerals, and most frequently the production of a new crystalline 

 structure." A fuller statement is given on page 764 : "Mere alteration 

 by decay is not what geologists denote by metamorphism. Tlie term has 

 been, indeed, much too loosely employed; but it is now generally used to 

 express a change in the mineralogical and chemical composition and in 

 the internal structure of rocks, either locally, by intruded masses of highly 

 heated material, or regionall}', through the operation of mechanical move- 

 ments, combined with the influence of heat and heated water or vapor." 

 However, Geikie does regard mere "induration" of discrete materials as 

 a metamorphic process. 



Geikie adopted the dicbotomous division into contact metamorphism 

 and regional metamorpliisra. He recognizes deep burial as one of the 

 causes of metamorphism, but decides (page 805) that the "statical phase" 

 of regional metamorphism is "not so striking in results as dynamical 

 metamorphism." 



