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Coal Basin is a deeply eroded anticline of <br /> some 26 square miles in size , which is rimmed by the Mesa- <br /> verde formation . The mine portals are situated on the west <br /> and southwest sides of the basin . The coal zones dip tc the <br /> west and southwest . <br /> b . Structure <br /> The structure of the Coal Basin is thought to <br /> have been formed by the doming of sedimentary strata over an <br /> igneous intrusion, probably a laccolith . The intrusion <br /> superimposed the Coal Basin, an anticline , on the Grand <br /> Hogback, a sharply folded monocline which resumes its <br /> generally north-south trend beyond the limits of the basin . <br /> The major axis of the Coal Basin plunges <br /> `165 , while the Grand Hogback dips westward . Eastward <br /> plunge is difficult to detect in the Coal Basin, probably <br /> due to the merging of the structure with the Grand Hogback, <br /> and to the erosion of the Mesaverde sediments and erratic <br /> attitudes of the Mancos Shale . Cretaceous Marcos Shale and <br /> the Cretaceous Mesaverde Group constitute the principal <br /> exposures of the Coal Basin . <br /> The main igneous masses are not exposed at <br /> the surface, although several zones of Tertiary age dacite <br /> dikes have been traced , both on the surface and underground . <br />