An orthoquartzite occurring in the Miocene Kirkwood formation of New Jersey is cemented by opal and chalcedony and contains fossils that are replaced by opal, chalcedony, and quartz. The rock is further characterized by its variable cementation, fine to medium sand size, light color, good sorting, and heavy mineral fraction which is similar to that of the unconsolidated portions of the Kirkwood and Cohansey formations. Evidence is presented to support the theory advocated by W. A. Tart (1926) that precipitated silica will in time pass from opal to chalcedony to quartz. It is suggested that the silica replaced the original shell material and then was precipitated between the grains as a cement after the sand was shifted by emergence from a neritic to a lagoonal environment.

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