Dolomite is forming in Deep Springs Lake, California, and marine-associated Coorong lakes of South Australia. Dolomite forms via a surface-layer precursor, which is commonly calcium-rich in comparison

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with the main body of the crystal, and which yields a calcium-rich dolomite by solid-state diffusion. This can be demonstrated to be true of most dolomite which forms in the absence of extraneous phases of calcite and magnesian calcite, from both the marine-associated Coorong lakes and Deep Springs Lake, by employing the following methods: (1) progressive leachings, with attendant chemical analyses; (2) X-ray diffraction; and (3) electron microscopy. X-ray diffraction data show that it is also true of dolomite forming in the presence of such extraneous phases. Dolomite also forms in the presence of magnesite or calcian magnesite, in certain of the marine-associated Coorong lakes; such dolomite is slightly magnesium rich. This dolomite probably forms via a magnesium-rich precursor, dep sited as a layer on the individual crystals of dolomite, in much the same manner as the calcium-rich layer on the calcium-rich dolomite. Carbon-14 dates of the calcium-rich dolomite from Deep Springs Lake yield growth rates of 500-900 angstroms/10³ years.

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