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thickness , from a few inches to 50 feet or more ; in composi- <br /> tion, from dacitic to andesitic ; and in texture, from <br /> athanitic to coarsely prophyritic . The dips are usuallv <br /> within a few degrees of vertical . <br /> Several zones of dacite dikes have been encoun- <br /> tered in the the Coal Basin mines . These rocks are reported <br /> to be of Tertiary age, and possibly associated with the <br /> igneous mass underlieing Coal Basin. <br /> Faulting is common and can be classified into <br /> four general groups : parallel to the major anticlinal axis ; <br /> perpendicular to the axis ; 300 off the axis ; and 600 off <br /> the axis . Faults 300 and 600 off the axis occur most fre- <br /> quently . Displacements are variable, from 20 ' or less to <br /> more than. 450 ' . Larger faults can be observed on the sur- <br /> face where prominent stratigraphic units have been inter- <br /> rupted , and are often associated with a marked change in the <br /> attitude of bedding . <br /> The two faults with greatest relative dis- <br /> placement are located on the north fork of Coal Creek (400 <br /> ft) and Porcupine Creek (300-600 ft) running parallel to the <br /> axis . Faults , in general , are more abundant and displace- <br /> ments greater in areas of significant strike change , such as <br /> the No . 4 mine area , and areas drained by the headwaters of <br /> Oilwell Creek, the north fork of Coal Creek, and Porcupine <br /> Creek . <br /> While the dikes do not as a rule follow <br /> 3 <br />