308 J. D. TRUEMAN 
2. Mesnard sericite schist. Column 2 represents the composi- 
tion of the sericite schist which occurs at the base of the Mesnard 
quartzite in the Marquette district of the Lake Superior region. 
This has been recently described’ as having been probably formed 
from quartzite by loss of silica, the other oxides being present in 
the same proportion in the two rocks. ‘Tests made on these rocks 
by the writer showed that zircon, while present in each case, 
was not sufficiently abundant to enable conclusions to be drawn 
regarding chemical change during metamorphism. 
THE CHARACTER OF THE CHEMICAL CHANGES DURING THE DEVELOP- 
MENT OF FOLIATION 
With regard to the nature and importance of the chemical 
changes which take place during the development of foliation it 
is only possible, at the present time, to indicate a few suggestions. 
Needless to say, the final composition is dependent on many factors, 
the principal of which are: the original mineralogical and chemical 
composition of the rock, the intensity and duration of dynamic 
action, the depth of burial and the proximity to igneous intrusions. 
In the absence of igneous activity, the process of alteration seems 
to favor the production of a composition determined largely by that 
of certain platy minerals which are relatively stable under the con- 
ditions of differential pressure. The character of the platy minerals 
which form seems to depend to a marked degree upon the original 
composition of the rock, e.g.,a talcschist appears to be the usual meta- 
morphic product of a limestone as sericite schist 1s of a quartzite. 
In the case of the quartzite, by the way, the writer’s observations 
suggest that with very intense metamorphism the sericite and 
iron oxide present in the less altered rock may combine, leading 
to the formation of a biotite schist much darker in color than the 
original. 
To illustrate the variation in the amount and direction of 
chemical change for any oxide in the case of rocks of different com- 
position, the SiO, content may be considered. The platy minerals 
mentioned in the last paragraph, being all silicates, have a some- 
what limited range in the SiO, percentage. It is, accordingly, to 
tU.S.G.S. Mono., LIT (1911), 257. 
