ABSTRACT

Oolites whose nuclei seem to consist of enhedral quartz crystals are found in a limestone bed of the Hale formation, lower Pennsylvanian age, near Fayetteville, Arkansas, The true character and origin of these nuclei are manifested most clearly in thin section if lighted by a darkfield condenser. The nuclei consist of two parts. The inner part has the size and shape of ordinary sand grains. The outer part, which bears the crystal faces, contains a faint ghost structure that (1) resembles and connects with the radial and concentric structure in the concretionary zone of the oolite, (2) conforms to the irregular shape of the inner part of the crystal, and (3) is obviously a residue of previously existing calcareous oolite structure. The inner part was originally a quartz sand grain that served first as the nucleus of the calcareous oolite and again, at a much later time, as the nucleus for the silica that replaced a part of the calcareous oolite. The source of the silica is obscure.