236 The American Geologist. ^p"1> ^^04. 
From the diagenic changes in a rock mass, it is but a step 
farther in the same direction to those changes produced by 
metamorphism as ordinarily understood. Whether the meta- 
morphism be due to contact with heated igneous masses (con- 
tact or thermo-metamorphism, or acthohalisin'^ ) or to moun- 
tain making processes (regional or dynamo-metamorphism, 
or syuiphrGttisuf\) the process, like diagenism must be con- 
sidered as primarily a modifying one, and moreover one far 
more restricted in activity than diagenism. Nor can the pro- 
duct of such metamorphism be considered as a class apart from 
the original rock mass, any more than the product of diagen- 
ism. In other words, as Walther has maintained^ the meta- 
morphic derivatives of rocks must be classed with the original 
recks in any classification based on genetic principles. 
Walther has embodied the principles of genetic classifica- 
tion in a number of theses, of which those dealing with the 
present problem may be cited : 
"Every ancient rock mass has primary characteristics, ap^ 
pearing at the formation of the mass, and secondary character- 
i='tics acquired through diagenism and metamorphism. 
"These characters, developed at different times, may change 
the type of the rock to such an extent that the secondary char- 
acters appear to be the principal and the primary characters 
the accessory ones. 
"In spite of this, primary characteristics alone determine 
the principal divisions of the lithologic system. 
"Characters acquired through chemical diagenesis or through 
contact or pressure metamorphism, must serve as secondary 
means for the determination of smaller groups. 
"The altered rock masses are to be placed with the original 
types." II 
To sum up, metamorphism may be considered as pre- 
senting itself under these phases, i. Static metamorphism or 
diagenism, the product being diagenic rocks ; 2. Contact or 
* From (I'.Oo'i^ a burning heat, and {j<l.KKo), to strike. Rocks of this type 
may be called aethobalic rocks. 
t From auiJ.<fj)dz7ii).^ to prt ss together. Rocks of this type may be called 
symphrattic rocks. 
t Vcrsuch einer Classification der Gesteine auf Grund der vergleichenden 
Lithogenie, Congr. Geol. International, Coinpte rendu de la VII session, St. 
Fetersbourg, 1S97, 3nie partie p. 9-23. 
II Versuch einer Classification, etc., p. 13. See also K¥.mt>, Handbook of 
Rocks, 2nd Edition, p. 11, where the same position is maintained. 
