Despite the long and continued interest of sedimentary petrologists in the provenance of ancient sediments, little is known about the detrital modes of Holocene sands of known parentage. As a result it has been impossible to use the "present as a key to the past" for purposes of provenance interpretation. In order to fill this information gap and to aid in further understanding of the provenance problem, first-cycle fluvial sands derived exclusively from granitic plutons in the relatively humid Appalachian Mountains and in the relatively arid Rocky Mountains in the United States have been studied. Characterization of the climatic influence on these sands is based on the results of detailed petrographic modal-analyses of three or five size fractions of 87 sand samples. Distinctive size-dependent compositional trends exist in the modal percents of the four principal constituents of the sands; these are potentially useful in paleoclimatic interpretation. In all size fractions rock fragments are more abundant in the arid west than in the humid east. Also, though rock fragments decrease in abundance with decreasing grain size in both the west and east, the rate of decrease is much higher in the west than in the east. Feldspars increase in abundance with decreasing grain size in both regions, but the quartz/ feldspar ratio is >1 in the east and <1 in the west. This relation holds for all size classes. Accessory, minerals are much less abundant than the feldspars, but show similar trends. Quartz generally decreases in abundance with decreasing grain size in the east. In the west quartz is more abundant in medium sand than in other size fractions. A schematic model is proposed to depict evolution of sand in arid and humid climates from polymineralic coarse-grained source rocks. The model predicts the relative abundances of the four principal detrital ingredients of first-cycle sands in different size fractions as a function of their relative resistance to weathering. This study also establishes the original mineralogic maturity of first-cycle sands. "Tie-lines" are drawn between coexisting sand compositions with similar maturity using a modification of the traditional QFR triangular diagram. In addition, trends toward mineralogic maturity within the framework of such a QFR diagram are defined.