Comparison of the carbonate mineralogy of sand, coarse silt, fine silt, and coarse clay fractions of river sediments from the Hudson Bay region demonstrates that dolomite is more abundant than calcite in the coarse silts but is less abundant than calcite in other size fractions. In coarse silts, dolomite commonly occurs as single crystals in the shape of well defined rhombs, whereas calcite is present as equant to subequant grains (few rhomb-shaped crystals). Dolomite grains appear to have greater resistance to abrasion than calcite grains after particles have been reduced to sizes lying in the coarse silt range. Although differing physical properties may play a role, it appears likely that the primary control on the particle size distributions of the carbonate minerals is their original crystal size in the source bedrock.The calcite/dolomite ratio for dolomite-enriched coarse silt parallels the regional distribution observed for sand, fine silt, and coarse clay, with highest values occurring in the Cape Henrietta Maria area. Carbonate mineral ratios for silt and clay fractions corroborate the results obtained for sands and provide further evidence for the existence of a carbonate dispersal train extending southwest of the cape. These results further demonstrate that fluvial carbonates can be useful indicators of regional drift dispersal and ice-flow patterns.The widespread use of silt or combined silt and clay fractions for calcite and dolomite determinations in drift provenance studies is not upheld by the results of this investigation. Coarse silts, which have been enriched in dolomite due to abrasion processes, provide a substantially distorted interpretation of source bedrock composition. Use of the sand-size grade is advocated. Sands, unlike coarse silts, reflect the major occurrence of limestones and minor dolostones in Paleozoic basins of the region.