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Abstract Detrital dolomite is an important part of stream-laid Cretaceous Comanchean rocks near the Llano uplift of central Texas. Modern stream deposits in and near the uplift also contain detrital dolomite which is derived from Ordovician and Cretaceous sucrosic dolomites. Dolomite grains in ancient rocks appear in thin section either as fragments of microcrystalline rock or as brown, angular to well-rounded central portions of dolomite crystals. In porous sandstone, the crystals have a clear sparry rim in optical continuity with the central grain; this rim is cement that formed as an overgrowth.
David L. Amsbury (Thu,) studied this question.