Analysis of zircons separated from nine samples of Precambrian rocks in the interval 786 to 2902 m, along geothermal test hole GT-2, reveals the presence of large percentages of light-colored as well as some dark brown grains in each sample. A majority of the zircons in all samples are rounded or subrounded, and a large number in all but two samples exhibit overgrowths. Some zircons from all samples possess extinction angles, many exhibit zoning, and a small percentage show truncated zoning. Zircon mean lengths and mean widths fall within narrow ranges. Mean elongations cluster near 2.0, typically, and elongation histograms are primarily bimodal. Reduced major axes vary in position, length, and slope. Collectively, the mean elongations, rounding index, euhedrism, overgrowths, extinction angles, truncated zoning, and reduced major axis data suggest that these zircons are detrital in nature. Thus the rocks in which they occur are inferred to be paragneisses.