CHEMICAL COMPOSITION IN SEDIMENTS 455 
rocks without important changes in chemical composition. It lends 
support to the view expressed by Rosenbusch that most foliated 
rocks of igneous origin have not suffered any great chemical changes 
during dynamic metamorphism. It does not prove, however, that 
important chemical changes may not take place in some igneous rocks 
as a result of dynamic metamorphism. 
THE META-SEDIMENTARY FOLIATES 
Among the foliated rocks of meta-sedimentary origin the siliceous 
foliates may usually be recognized without detailed chemical studies 
from their highly quartzose character. Being more resistant than 
most other sediments to mass deformation they are also more likely to 
preserve during metamorphism traces of bedding and other original 
structures. Itis seldom therefore that chemical data need be appealed 
to for the recognition of sediments of this type. The calcareous 
foliates also are usually recognized readily from their high carbonate 
content without recourse to refined chemical study. 
It is evident, on the other hand, that the frangitic foliates, rocks 
like the arkose and graywacke-schists, which are the result of the 
disintegration of igneous rocks without much decomposition or 
mechanical sorting, will closely resemble their parent igneous rocks 
chemically, and that in such cases chemical criteria have little or no 
value for determining genesis. It is only in the recognition of the 
remaining division, the pelitic foliates, that chemical criteria are 
particularly useful, and it is here that they have been most frequently 
appealed to. It should be remembered, however, that the four types 
of sediments mentioned above are not sharply delimited but pass into 
each other through every conceivable gradation. It is not possible 
therefore to define the exact limits of usefulness of chemical criteria, 
though the major limitations outlined are believed to be essentially 
correct. 
The chemical characteristics of the metamorphosed pelitic sedi- 
ments can only be adequately studied by comparing a considerable 
number of analyses of slates, phyllites, and schists. 
No. I below represents the average of 79 slate and phyllite analyses, 
36 being from the United States and Canada and 43 from Europe. 
Only those of undoubted sedimentary origin and unaffected by contact 
