Sandstones in which silica forms the cementing material commonly exhibit secondary overgrowth, in optical continuity, on detrital quartz grains (Pettijohn 1957). A line of inclusions and/or a precementation limonitic coating usually marks the detrital boundary. Juxtaposition of secondary rims separately overgrown on several adjacent nuclei produces a sort of interlocking fabric. A conspicuous tendency shown by sandstones having a considerable proportion of silica cement is the formation of idiomorphic facets on the reconstituted grains (Fig. 1).