ABSTRACT

The quartz and feldspar content was determined for 70 stream sand samples from the South Platte, Platte, and Missouri rivers. Between the headwaters of the South Platte and Denver, Colorado, the changes in quartz and feldspar content are erratic, but between Denver and the junction of the Missouri and Mississippi rivers, quartz increases about 10 percent, potash feldspar decreases about 11 percent, and plagioclase feldspar increases about 1 percent. If these results are combined with those of Russell (1937) and Willman (1942) for the Mississippi River, quartz increases 19 percent, potash feldspar decreases 13 percent, and plagioclase feldspar decreases 3 percent between Denver and the Gulf of Mexico.

The average of 11 stream sands shows no variation of quartz content with grain size, but potash feldspar content decreases with decreasing grain size, and plagioclase feldspar increases with decreasing grain size. The average of 7 gruss samples shows the same trends; hence, weathering and the grain size of the various minerals in the original rock are believed to strongly influence the distribution of quartz and feldspar in the various size fractions. The difference between the quartz and feldspar content of the stream and gruss samples indicates that the feldspar content decreases quickly after the gruss material enters the stream.

Pettijohn's (1957) conclusions that the feldspar content of stream sands is reduced rapidly in high-gradient streams, but is lost slowly in low-gradient streams seems to hold in the South Platte, Platte, Missouri, and Mississippi river drainage system.