ORIGIN OF FOLIATED CRYSTALLINE ROCKS emis 
indeed, shows that practically all mica schists have, according to 
the criteria mentioned, the composition of sedimentary rocks. 
Probably the majority of such schists do represent altered sedi- 
ments although some have been described’ as originating from 
igneous rocks. It seems likely that if biotite in many igneous 
rocks were increased to 25 per cent, as was tested in the case of the 
Butte granite, with a corresponding decrease of other constituents, 
a sedimentary composition would be reached. Chlorite, which 
frequently forms the major part of some schists derived from igneous 
rocks also possesses a composition belonging to the sedimentary 
type rather than the igneous. 
Less, even, is known of the extent of the chemical changes than 
their character, probably because it has not been generally held 
that rocks suffer significant chemical changes during the develop- 
ment of foliation. The sericite schist associated with the Mesnard 
quartzite is an important rock formation, and the chemical changes 
involved in its formation from a quartzite would be enormous. 
The writer is of the opinion that while chemical change during the 
development of sericite schist from quartzite is very striking, less 
marked changes probably take place more readily in igneous or 
argillaceous rocks since the latter types more readily undergo 
differential movement, a process which seems essential for the 
production of schistosity. 
Proof of important changes in chemical composition during the 
development of foliation in certain rocks will not necessarily destroy 
the usefulness of chemical data as a criterion, though its limitations 
will become better recognized. The origin of igneous rocks which 
have not been greatly altered may, for example, possibly be recog- 
nized with some certainty from the chemical composition, since 
the process of alteration seems to tend toward that of the sedi- 
mentary type. 
GENERAL SUMMARY 
In the introduction to this article the writer has attempted to 
give a brief summary of the criteria which have been suggested for 
determining the origin of foliated crystalline rocks. These rocks 
tE.g., A. Keith, U.S.G.S. Folio 70, 1got, p. 2. 
