CHEMICAL COMPOSITION IN SEDIMENTS 449 
I. The presence of silica in certain schists and gneisses in larger 
proportions than in most igneous rocks. 
II. The presence in many of alumina in amounts considerably in 
excess of that necessary to satisfy the ratio of 1:1 in which it is 
combined with lime and the alkalies in the common rock-forming 
silicates. 
III. Dominance of magnesia over lime. 
IV. Dominance of potash over soda. 
In the following pages the writer purposes to show that foliated 
structures may be developed in many rocks without important 
changes in chemical composition, and proposes to consider in detail 
the value of each of the above criteria. Before discussing the criteria 
further it will be well to differentiate the types of foliated rocks and 
determine to which ones they can be appropriately applied. The 
foliated rocks may be classified as follows: 
FOLIATES* 
I. Primary foliates (flow-foliates or ortho-foliates)——Here belong the flow- 
gneisses and all igneous rocks whose foliated structure is original and is due 
to differential movements in the igneous magma before complete solidification. 
II. Secondary foliates (metamorphic-foliates or para-foliates)—Rocks whose 
foliated structure has been induced by metamorphism subsequent to their 
complete consolidation. 
1) Meta-igneous foliates: 
a) Meta-plutonic foliates. 
b) Meta-volcanic foliates. 
2) Meta-sedimentary foliates: 
a) Siliceous foliates. 
Quartzitic schists and other metamorphic derivatives of highly quart- 
zose sediments. 
b) Calcareous foliates. 
Banded marbles and other metamorphic derivatives of highly calca- 
reous sediments. 
c) Pelitic foliates. 
Slates, phyllites, argillaceous schists, and other metamorphic deriva- 
tives of argillaceous sediments. 
t The term ‘‘foliates” is here used as a convenient comprehensive term to include 
all rocks showing foliated structures other than bedding planes. Its use in a discus- 
sion of this kind saves frequent repetitions of the two terms, schists and gneisses, and 
avoids any postulate as to the primary or secondary character of the foliated structure. 
