Electron microprobe techniques were used to examine the distribution of Mg, Ca, and Fe in six samples of Mississippian skeletal and oölitic limestones from the Alberta Rocky Mountains. All samples show extremely low values of Fe. The following distributions of Mg (in wt % MgCO3) were observed: in four samples from the Loomis Member (Meramec) of the Mount Head Formation, echinoderm grains show highly variable Mg content averaging 1.11–1.43, whereas sparry calcite cement and the outer rings of oöids show much more uniform values averaging 0.73–0.84. In one sample from the Livingstone Formation (Osage), echinoderm grains show large Mg variation with a mean of 0.95; sparry calcite cement shows more uniform values with a mean of 0.50. A second, highly recrystallized sample from the Livingstone Formation has echinoderm grains which show large Mg variation with a mean of 0.43. The stroma canal structure characteristic of living echinoderms is not generally preserved in the grains. All of the echinoderm grains analyzed contain abundant dolomite crystals 1 to 6 μ in diameter which appear to account for the variable values of Mg distribution, but are not obviously related to the original stroma canal structure. Sparry calcite cements and oöid rings are free of dolomite crystals. The small size of the dolomite crystals, combined with their widespread distribution within and restriction to echinoderm grains, suggests an exsolution origin. Exsolution is suggested to have taken place subsequent to a) loss of most of the Mg from the original magnesium-rich echinoderm calcite, and b) precipitation of pore-filling calcite within the echinoderm grains. The minute dolomite crystals are thus interpreted as representing a late stage exsolution product of small residual amounts of Mg from both the original grains and their pore-filling calcite.