At the request of the U.S. Environmental Protection Agency (USEPA), the U.S. Geological Survey (USGS) has conducted an independent study of amphiboles in rocks and soils in the El Dorado Hills, California, area. The purpose of this study is to investigate specific issues regarding the presence of naturally occurring asbestos raised by an USEPA activity-based sampling study and subsequent criticisms of that study outlined in a review prepared by The R.J. Lee Group (RJLG). In their review, the RJLG challenged results of the USEPA study and suggested that the materials identified as asbestos by USEPA and its contract analytical laboratories do not meet the definitions of asbestos for the purposes of regulation and therefore should not be considered as a potential public health concern. The RJLG report suggested that amphibole asbestos was not present in USEPA's samples because (1) approximately 60 percent of the particles had too much aluminum to form asbestiform amphibole, (2) aspect ratios (length to width) of the particles did not represent a population of asbestos fibers, and (3) optical properties of the particles are not consistent with asbestos particles. For this study, samples from bedrock outcrops and soils were collected in the El Dorado Hills study area by USGS scientists and were analyzed by a variety of techniques in order to define chemistry, mineralogy, and mineral morphology.