A number of mechanisms have been proposed, including the dehydration of clay-rich, gel-rich, or organic-rich cores; shrinkage of the concretion's center; expansion of gases produced by the decay of organic matter; or brittle fracturing or shrinkage of the concretion interior by either earthquakes or compaction (Pratt 2001; McBride et al. 2003). Septaria usually contain crystals, often calcite, that precipitated from circulating solutions.