Microcline perthites from Precambrian pegmatites in the Front Range, Colorado, have been converted to orthoclase in a zone around a Tertiary intrusive stock. This stock, near the town of Eldor, is 2 miles across and shows fairly well exposed vertical contacts. The microcline-orthoclase transition, the only major manifestation of contact metamorphism, was investigated optically and by X-ray powder diffraction along five traverses. Outside the contact aureole and microcline commonly consists of clear crystals with typical crosshatched twinning and shows a 2V of 80° ± 5° and an obliquity ranging from 0.84 to 0.93. The orthoclase from near the contact is always turbid, untwinned, with axial angles ⊥ (010) from 50° to 66°. The perthitization and composition (Or77Ab23) of the feldspars remain roughly constant across the contact aureole, except within the last few feet from the contact, where an increased albite content or the disappearance of the perthite lamellae may occasionally be observed. Both the nature of the microcline-orthoclase transition and its distance from the contact depend strongly on the configuration of the contact in that particular area. In accordance with the heat flow calculations, the transition is relatively sharp and close (1200 feet or less) to protruding corners of the intrusive, whereas, near re-entrant sections of the intrusive, the transition is gradational and relatively distant (over 2400 feet) from the contact. Heat flow calculations further indicate that the microcline-orthoclase transition may occur at temperatures below 400°C, i.e. more than 50°C lower than was hitherto known from an experimental approach. The order of the transition is not specified by this investigation although the occurrence of some intermediate microcline in the transition zone may be evidence for a continuous type of transformation.