312 C. K. LEITH 
shales, may occur, while igneous rocks under the same conditions 
tend to katamorphism. In a highly idealized form the distribution 
of the changes with regard to zones, kinds of rocks, and kinds of 
changes, may be represented as in the figure on p. 312, in which the 
extent or importance of the change is represented in each case by 
thickness of line. 
Van Hise states that the processes in each of the metamorphic 
zones may be reversed, thus in a way recognizing the cycle principle, 
but his full discussion emphasizes the dominant conditions and 
changes of the zones to such an extent that the student sometimes 
infers that all rocks in a given zone act alike. Grubenmann des- 
cribed three zones of schist-making—upper, middle, and lower— 
with the implication that all rocks suffer uniform changes in a given 
zone. Chamberlin and Salisbury imply the cycle idea in their use 
of the terms “descensional” and ‘‘reascensional” as applied to rock- 
changes, though the account of alterations and conditions is neces- 
sarily elementary. It is the purpose of the present paper to emphasize 
the necessity of the cycle theory in interpreting the zones of meta- 
morphism. The “zonal” classification permits of necessary grouping 
of dominant phenomena, but confusion is likely to result unless the 
zones are considered in connection with particular rocks and altera- 
tions, and contrasting kinds of results for each zone are clearly dis- 
criminated. 
Réle of igneous rocks in metamorphic cycle-—The part taken in the 
metamorphic cycle by contact action of igneous rocks is most con- 
spicuous in the anamorphic or constructive phases of the cycle, and 
may be regarded in general as furnishing higher temperature and 
pressure and hot solutions for the acceleration of anamorphism and 
for developing anamorphism in higher zones where the conditions are 
otherwise not anamorphic. Along the contacts are developed dense, 
complex, anhydrous silicates, such as garnet, staurolite, andalusite, 
amphibole, pyroxene, etc., and the rocks are recrystallized. Yet it 
is also true, especially near the surface, that the advent of igneous 
rocks brings about katamorphism in the adjacent rocks through the 
agencies of the hot waters and gases, magmatic, meteoric, or both, 
accompanying and following their introduction. In this class prob- 
ably belong in part the prophylitic and allied alterations adjacent to 
