not as common as limestone. It is typically found in association with limestone or evaporite beds and is often interbedded with limestone. There is no consistent trend in its abundance with age, but most dolomite appears to have formed at high stands of sea level. Little dolomite is found in Cenozoic beds (beds less than 65 million years old), which has been a time of generally low sea levels. Times of high sea level also tend to be times of a greenhouse Earth, and it is possible that greenhouse conditions are the trigger for dolomite formation. Many dolomites show clear textural indications that they are secondary dolomites, formed by replacement of limestone. However, although much research has gone into understanding this process of dolomitization, the process remains poorly understood. There are also fine-grained dolomites showing no textural indications that they formed by replacement, and it is uncertain whether they formed by replacement of limestone that left no textural traces or are true primary dolomites. This dolomite problem was first recognized over two centuries ago but is still not fully resolved. The dolomitization reaction is thermodynamically favorable, with a Gibbs free energy of about -2.2 kcal/mol. In theory, ordinary seawater contains sufficient dissolved magnesium to cause dolomitization. However, because of the very slow rate of diffusion of ions in solid mineral grains at ordinary temperatures, the process can occur only by simultaneous dissolution of calcite and crystallization of dolomite. This in turn requires that large volumes of magnesium-bearing fluids are flushed through the pore space in the dolomitizing limestone. Several processes have been proposed for dolomitization. The hypersaline model (also known as the evaporative reflux model) is based on the observation that dolomite is very commonly found in association with limestone and evaporites, with the limestone often interbedded with the dolomite. According to this model, dolomitization takes place in a closed basin where seawater is subject to high rates of evaporation. This results in precipitation of gypsum and aragonite, raising the magnesium to calcium ratio of the remaining brine. The brine is also dense, so it sinks into the pore space of any underlying limestone (seepage refluxion), flushing out the existing pore fluid and causing dolomitization. The Permian Basin of North America has been put forward as an example of an environment in which this process took place. A variant of this model has been proposed for sabkha environments in which