In connection with field-work undertaken for the Colorado Geological Survey the past summer in the vicinity of La Junta, Colorado, the author has had occasion to note a gradual transition or merging of two formations that heretofore have been considered as belonging to two different groups, the Benton and the Niobrara groups. The present communication is presented with the consent of the Director of the Survey.
In southeastern Colorado the Benton group is developed as three readily distinguished formations. The lowest of the three is the Graneros shale, with some 200 feet of gray to black shales. The highest, or Carlile shale, has usually about the same thickness and is likewise composed of dark gray shales, which at the top change to a yellowish sandstone of 20 feet thickness or over. The. two are separated by some 50 feet of Greenhorn limestone. The Niobrara group has a twofold division— . . .