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basin, known as Huntsman Ridge . Lands which overlie the <br /> workings are the west and south slopes of Huntsman Ridge. <br /> Coal Basin is thought to have been formed by <br /> the doming of sedimentary strata over an igneous intrusion, <br /> probably a laccolith. The intrusion superimposed the Coal <br /> Basin anticline on the Grand Hogback, a sharply folded <br /> monocline which resumes its generally north-south trend <br /> beyond the limits of the basin. <br /> The major axis of the Coal Basin plunges <br /> N65W, while the Grand Hogback dips westward . Eastward <br /> plunge is difficult to detect in Coal Basin , probably due to <br /> the merging of the structure with the Grand Hogback, and to <br /> the erosion of the Mesaverde sediments and the erratic <br /> attitudes of the Mancos Shale . Cretaceous Mancos Shale and <br /> the Cretaceous Mesaverde Group constitute the principal <br /> exposures in 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 /> In addition, drilling within the basin area has revealed <br /> igneous rock at depth, thought to be the intrusive core of <br /> the arch. <br /> The dikes occur south of Coal Creek and <br /> generally strike from nearly east-west to N30W , with many <br /> local variations . Where dikes or sills have intruded coal <br /> seams , natural coking has occurred . The dikes vary ; in <br /> 2 <br />