Data, indicating that contraction commonly takes place in cherts as they pass from a stiff gel into the true opaline phase, have been collected by the writer in the course of field work in California during a number of years. Microscopic examination of the purer cherts, especially those of the Miocene, indicates that rather irregular contraction has taken place in many instances. Although this irregular contraction toward dispersed centers is perhaps the most universal type, there are certain regular megascopic forms that are probably of greater general interest. These assume the form of concentrically banded oblate spheroids, the bands being of ordinary chert alternating with chalcedony or, more rarely, almost pure opal. The relatively pure bands of chalcedony (or opal) are often spaced with truly remarkable regularity and produce a striking effect. Such spheroids are not confined to the marine Monterey and Franciscan cherts but occur also in certain . . .

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