Modern X-ray diffractometer, optical, and chemical techniques coupled with principles developed by recent phase-equilibria studies of the hydrous systems Ab-Or (Bowen and Tuttle, 1950), Ab-Or-Q (Tuttle and Bowen, 1958), and Ab- Or-An (Yoder, Stewart, and Smith, 1957) have been applied to the feldspars from 68 analyzed rocks belonging to a complex comagmatic series, the Laramide in trusive sequence in the Colorado Front Range. Consistent differences in composit ion, thermal state, and paragenesis of the feldspars confirm the subdivision of the rocks into two related lines of descent, based upon bulk composition and mafic mineralogy, and throw light on the...