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COMPOSITION OF METAMORPHOSED SEDIMENTS 201 
fracture zones which later became the locus of shearing movements. 
All that is required to produce the chemical changes observed is 
the removal of silica by waters circulating along such fractures. 
It is also impossible in this region to eliminate the possible effect 
of igneous rocks as a source of energy which might result in such 
changes. The Waterloo quartzite outcrops only in small ledges 
separated by large areas covered by glacial drift. Pegmatites 
which appear to be earlier than the shearing of the quartzite cut 
this rock in certain of the ledges. Although no pegmatite outcrops 
at the locality where the samples analyzed were taken, it is impos- 
sible to say that it does not occur close by beneath the drift cover. 
Whatever may be the facts in regard to the mobility of silica 
under certain conditions, it is beyond question that in the meta- 
morphism of many sedimentary series the quartzite members 
preserve their integrity even when the associated argillaceous beds 
are rendered highly schistose, nor do the latter show any marked 
accession of silica. It has been suggested that a talc schist is 
the characteristic metamorphic equivalent of limestone as a 
sericite schist is of quartzite, but here again we are confronted with 
numerous field observations showing abrupt passages from crystal- 
line limestones to argillaceous schists and to quartzites, even in 
severely metamorphosed series. Furthermore, with the possible 
exception of the expulsion of silica, the development of platy 
minerals during dynamo-metamorphism would seem to be a process 
ill suited to the accomplishment of any considerable changes in 
chemical composition, for the platy minerals that are characteris- 
tically developed in schists during dynamo-metamorphism are 
minerals of extremely variable composition, and the same elements 
enter into the makeup of a number of them. For this reason they 
are especially adapted to accommodate original differences in 
composition both through variation in the relative proportions of 
the different platy minerals developed and through variations in 
composition of each of these minerals. 
WASHINGTON, D.C. 
December, 1912 
