A field and petrologic study of the Lone Grove granitic pluton and surrounding rocks from the Llano Uplift, Texas, suggests that this area has been involved in a single orogenic cycle with no later general metamorphism. Samples of granite, aplite, pegmatite, rhyolite, and metamorphic rocks were investigated in order to determine the precision in Rb-Sr and K-Ar ages between various minerals and different localities. Refined chemical and mass spectro-metric methods are capable of yielding ages on most highly radiogenic minerals to an analytical precision of ± 1 ½ per cent or better. Most of the ages from these rocks show a spread commensurate with the experimental error.