The combined basal conglomerates of the Antlers and Twin Mountains formations (Cretaceous) in north-central Texas are interpreted as a paleo-silcrete horizon. Zoned, repetitious siliceous cements developed within the intergranular porosity in a sequence from checkerboard chalcedony, through isopachous and spherulitic chalcedony, to drusy megaquartz. This sequence may be truncated or interrupted but tends not to reverse or repeat. The regional distribution of this sequence, the ubiquity of this sequence in published reports, and observations that the chalcedonic cements are all composed of minute quartz crystals suggests that, in this case, the silica speciation is a function of the rate of formation as determined by the hydraulic conductivity of the conglomerate during cementation and hence the availability of the silicifying fluid.

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