Twenty of the twenty-eight mudstone and shale samples investigated from the Upper Pennsylvanian Madera Formation near Albuquerque, New Mexico, can be divided into two groups showing two distinct parallel isochrons with different initial 87 Sr/ 86 Sr ratios by the Rb-Sr method of dating. The table below shows this. All errors are one standard deviation. Table The Rb-Sr isochron plots of data of the remaining eight samples show scatter about an 859 + or - 68 m.y. isochron with initial 87 Sr/ 86 Sr = 0.7010 + or - 0.0023. The samples which define parallel isochrons are composed dominantly of 1M and 1Md illites while those showing scatter contain an abundance of 2M 1 illite. The age of the Madera Formation is Upper Pennsylvanian (Szabo, 1953). The 255 m.y. and 264 m.y. dates reported here are nearly identical within limits of error but the latter is more precise due to smaller size fraction sample ( i.e. , removal of detrital phases) and smaller analytical error (all samples were analyzed by isotope dilution) and therefore, is considered closer to the age of sedimentation. The 264 m.y. whole-rock date agrees within limits of error with the Pennsylvanian-Permian boundary age (270 + or - 5 m.y.) of Holmes (1960). This boundary in the area of present report is controversial due to absence of depositional hiatus. It is proposed that this boundary is at the top of the Madera Formation. A model age of 1,430 m.y. has been calculated for the source rocks of the Madera Formation. Available evidence indicates that this model age is reasonably close to that of the Precambrian rocks of the Pedernal Hills which probably were the major source of the detritus of the Madera Formation.