The Spanish Peaks region in Colorado is famous for its remarkable system of radial dikes. These dikes and the accompanying sills were mapped by R. C. Hills in the Spanish Peaks and Walsenburg folios of the United States Geological Survey. The dikes extend out from the Spanish Peaks as a center to a distance of 25 miles or more. Some 300 dikes were mapped by Hills; but, inasmuch as he mapped only two-thirds of the area surrounding the Peaks, and, of necessity, only the thicker and more prominent dikes, the total number of dikes is probably not less than 500. Hills’ work was done mainly for the purpose of determining the important coal resources of the area, and, in view of the inadequacy of the topographic maps (1:125,000), the mapping of the dikes stands as a remarkably fine achievement. According to Hills, the dikes were injected in the . . .