314 J. D. TRUEMAN 
mentary origin but is only suggestive when other evidences are 
lacking; absence of zircon grains in quartzose layers indicates 
that such material is probably not of sedimentary origin but was 
deposited from solution; similarity in zircon grains may be used 
to establish the identity of altered rocks when the fresh types are 
available. 
Chemical analysis has frequently been used as a means of deter- 
mining the original igneous or sedimentary character of meta- 
morphic rocks. Its use is based on the supposition that a rock, 
as a whole, undergoes no significant chemical change during the 
development of schistosity. C. K. Leith has recently suggested 
that such chemical changes may be important and are probably 
controlled by the composition of certain platy minerals. It has 
been his view that material unsuited for the formation of such 
minerals tends to become removed during the development of 
foliation. The proof of such chemical changes is largely based on, 
first, field observations which seem to show that in certain rock 
alterations there must have been changes in chemical composition 
on account of the mineralogical composition of the altered and 
unaltered rocks, and, second, pairs of analyses of fresh and altered 
rocks in which the proportion between the oxides necessary for 
the formation of the platy minerals have remained constant while 
the other oxides have usually decreased in amount in the altered 
rock. The writer introduces new evidence by the consideration 
‘of the percentage of zircon in fresh and altered rocks. Zircon, as 
has been shown before, generally remains unaltered during the 
development of foliation. The case of the quartzite at Waterloo, 
Wis., is discussed in some detail and it is shown, both by means of 
mechanical separation. and chemical analysis, that the zircon 
content of the schist is much greater than that of the quartzite. 
Reasons are given for believing that the schistose bands do not 
represent argillaceous layers in the quartzite. The evidence of 
the zircon is supported by that of ilmenite which is also more 
abundant in the schist than in the quartzite. A short review of 
the mineral composition of platy, minerals indicates that an increase 
of such minerals in a rock would lead to a composition belonging 
to the sedimentary type according to the criteria of Bastin. This 
