Contrasting differences in rates of solution of calcite and dolomite in hydrochloric acid permit one to differentiate them and to determine their percentage in finely ground samples (minus-200-mesh), by taking 2 readings of the value of the CO 2 evolved in the Chittick gasometric apparatus. The minus-200-mesh grade of clastic sediments or crushed rock is used for this analysis, and 425, 830, or 1700 mg are taken, depending upon the carbonate content of the sample. The probable error of the results, which are expressed as percentages of carbonates from the total sample analyzed, does not exceed + or -0.3 in carefully done analyses. One analysis usually requires approximately 15-45 minutes; the time spent on preparing each sample, setting the apparatus, taking readings and computing the results, is approximately 10 minutes. Although the procedure was developed while studying Pleistocene clastic deposits, it can be applied to any rocks or soils containing calcite and dolomite. An unusual dolostone, which dissolves in hydrochloric acid more rapidly than most dolomite, has been found among the Devonian Detroit River Group in the Huron basin of southwestern Ontario.