THE WORLD IN A GRAIN OF SAND

            The detrital composition of modern sand is equally or more diverse than that of ancient sandstone as shown by conventional thin sections.  On Hawaii (the Big Island), for example, beaches may be 100% carbonate alochems, 100% basalt or basaltic glass, or 100% olivine.  Other essentially "monomineralic" sand includes the beaches on other basaltic coasts (e.g., Iceland, Bay of Naples, Tahiti), gypsum sand of White Sands National Monument in New Mexico, oolite sand of Great Salt Lake in Utah, quartzarenite beaches of the Florida Panhandle, and the graywacke-arenite beaches of South Island, New Zealand.  Modern sands are rich in anthropomorphic components: colored glass grains from containers; limestone and marble title downstream from tile factories (Italy); "tar" balls from evacuated ship's bilges; oil seeps; angular pieces of steel shrapnel on the Normandy beaches of France; and so on. 

            Modern sands frequently have forensic value.  An example: authorities asked McBride whether a murder suspect in Texas had been to the Rio Grande, the border with Mexico.  A thimbleful of sand collected from the suspect's pants cuff was in hand.  The heavy mineral suite matched that of the Colorado River at Austin, Texas, which drains granitic and limestone terrain, and was not sand of the Rio Grande, which passes through volcanic lands. 

            The average composition of "big river sands" (Potter, 1978) is Q60F12R28, but the sand of even one river can range widely because of sidestream input.  Colorado River sand downstream from Lees Ferry, AZ, ranges in composition from Q12FOR88 to Q60F12R28; limestone rock fragments from 2 to 50%, shale from 0 to 55%.