In the analysis of the average igneous rock the alkalis generally represent less than ten per cent of the total constituents that are present. The standard methods for their determination all call for the separation and weighing of the alkalis in the form of the combined chlorides. Potassium is then generally separated and determined gravimetrically while sodium is obtained by difference. In a great many cases the total aklalis are present in such small amounts that it is impossible to accurately separate them so they are often reported as Na2O+K2O. The following method for the determination of sodium and potassium is based upon the the microscopic determination of the index of refraction of a fusion of the combined chlorides and is, therefore, equally well adapted to either large or small precipitates. The maximum error should be less than 2 per cent and in many cases should be well under one per cent. In an analysis in which the total alkalis represent 10 per cent of the total constituents this would give an error of 0.2 per cent on the whole sample, and in cases where the alkalis represent 3 per cent or less the error would be under 0.06 per cent on the whole sample.