Calcareous cone-in-cone structures (Fig. 2) are spectacular features that have intrigued scientists since their discovery in the 19th century. Bates and Jackson (1987) provided the following definition: "A minor sedimentary structure in thin, generally calcareous layers of some shales and in the outer parts of large concretions, esp. septaria; it resembles a set of concentric, right circular cones fitting one into another in inverted positions (base upward, apex downward), commonly separated by clay films and consisting usually of fibrous calcite and rarely of siderite or gypsum...". Rare cone-in-cone-like structures consist also of pyrite (Carstens 1984), gypsum or siderite (Mozley 2003), but in these cases
(Pettijohn 1957) their characteristics and origin appear to differ significantly from those of the calcareous variety. The following description of cone-in-cone structures is based mostly on the works of Gresley (1894), Tarr (1922, 1932), Gilman and Metzger (1967), Franks (1969), and Selles-Martinez (1994).
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