ABSTRACT

Samples of thoroughly disintegrated granite from a granitic knob (Sandstone Mountain) in the Llano area of central Texas and from Flagstaff Mountain, Colorado, exhibit bulk sample size distributions which follow Rosin's law of crushing. This same distribution is also found in samples which have traveled several hundred yards down a small intermittent stream draining Sandstone Mountain. Less completely disintegrated granite samples from Bear Mountain, Texas, and Flagstaff Mountain depart from Rosin's law in showing a deficiency in the finer sizes. Hornblende grains weathered out of the granite at Sandstone Mountain without fracturing and zircon grains in weathered granite at Flagstaff Mountain exhibit the lognormal distribution which they probably had in the fresh granites.

Mineralogy and grain size are closely correlatable in the disintegrated granites from Texas. The percentage of quartz and rock fragments increases with increasing grain size whereas the percentage of the feldspars and heavy minerals decreases. The tendency of quartz and rock fragments to be more spherical than feldspar causes sphericity to increase with grain size. The decrease of sphericity with decreasing grain size is shown both in the samples from the Llano area and from Flagstaff Mountain.