Two new chemical analyses show that the varved argillites of the Precambrian Cobalt series of Ontario, Canada, contain more Na2O than K2O and have a Na2O/K2O ratio higher than most pelitic rocks but similar to that found in many graywackes. Published chemical data substantiate these observations. The Cobalt materials are also low in CaO but otherwise have a composition compatible with their presumed glacial origin.

Albitization of the detrital plagioclase feldspar seems to best account for the low lime and high soda content of these rocks. X-ray data confirm the presence of albite.

Spectrographic analyses of the argillites are also given.

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