CHEMICAL COMPOSITION IN SEDIMENTS 459 
never been in the belt of weathering have not been affected by the 
same kinds of chemical changes. 
Third, Both pelites and igneous rocks undergo chemical changes 
during dynamic metamorphism, but in many if not in most cases, 
these are not of sufficient magnitude to obscure the original igneous 
or sedimentary characters. The nature and value of various chemical 
criteria may therefore be determined by a direct comparison of the 
pelites with the unaltered igneous rocks as tabulated in Washington’s 
tables. The changes that do take place in igneous rocks during 
metamorphism appear to be of lesser magnitude and of a different 
character from those which affect the sediments. Professor C. K. 
Leith in a letter to the writer has formulated the conception that 
under conditions of anamorphism, both igneous and sedimentary 
rocks tend toward a common rock type characterized by a few platy 
and columnar minerals such as mica, chlorite, talc, hornblende, etc., 
and that constituents in excess of the proportions necessary to the 
formation of these minerals will be driven off. To the writer’s mind 
this also involves the conception of a possible addition of similar 
materials to other rocks where their presence may facilitate the devel- 
opment of such minerals. The conception is an interesting and 
valuable one. From the theoretical standpoint it is necessary to 
admit the possibility of profound chemical changes as a result of 
prolonged dynamic metamorphism, and it seems indeed probable 
that certain rocks now exposed in the older terranes may have been 
affected by such processes. The point particularly pertinent to the 
present discussion is the extent to which such equalizing action has 
gone on among most of the metamorphic rocks with which the 
geologist has to deal. The evidence already cited seems to the writer 
to indicate that complete recrystallization with the development of 
perfect foliated structures may take place in both igneous rocks and 
pelitic sediments, without changes of sufficient magnitude to obscure 
their original sedimentary or igneous character. Changes in silica 
content are a possible exception. 
The possibility is fully recognized, however, that in certain cases 
prolonged and severe dynamic metamorphism may produce changes 
in igneous or sedimentary rocks which render chemical differentiation 
between them difficult or impossible. 
