Mineralogic and chemical data have been obtained for 26 argillite samples collected from an 8500 meter section of the Precambrian Belt Supergroup, Clark Fork, Idaho. Minerals present are quartz, feldspar, illite, and calcite + or - calcite and/or dolomite. Illite/quartz and chlorite/quartz ratios decrease up-section supporting the conclusion of Boyce (1973) that the Belt section represents an ancient prograding delta. The ratio of K-feldspar to total feldspar decreases from 20-80 percent in the upper two-thirds of the section to 0-10 percent in the lower one-third. The mechanism proposed for the decrease in K-spar down-section is the conversion of K-poor smectite into K-rich 1Md illite. A later higher-temperature reaction converted much of the 1Md illite into 2M illite (muscovite). We suggest that a low K-spar/plagioclase ratio in an illite-containing shale suite indicates that the illite formed from smectite during metamorphism rather than from the direct weathering of some high-temperature phase such as feldspar. Most continental volcanic material is produced during orogenic episodes. Most smectite forms from volcanic material and smectites will be converted into illites if enough time has elapsed for them to have been involved in one or more geosynclinal cycles. Therefore, the proportion of smectite in sediments through time can be related to the timing of orogenic episodes. This model works fairly well in North America where smectite maxima coincide with orogenic episodes, becoming smaller with increasing antiquity of the orogeny.