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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 -:ary in <br /> thickness , from a few inches to 50 feet or more ; in composi- <br /> tion, from dacitic to andesitic ; in texture , from athanitic <br /> to coarsely porphyritic . The dips are usually within a few <br /> degrees of vertical . <br /> Faulting is common and can be classified into <br /> four general groups : parallel to the major anticlinal axis ; <br /> 0 0 <br /> perpendicular to the axis ; 30 off the axis ; and 60 off the <br /> 0 0 <br /> axis . Faults 30 and 60 off the axis occur most frequently . <br /> Displacements are variable , from 20 ' or less <br /> to more than ' 50 ' . Larger faults -an be observed on the <br /> surface where prominent stratigraphic units have been inter- <br /> rupted , and are often associated with :narked change in the <br /> attitude of bedding . <br /> While the dikes do not as a rule follow <br /> faults , there is an association in the first , second and <br /> third norrh entries of the Dutch Creek No . 2 mine entry , and <br /> in the drainage of Oilwell Creek between the L . S . Wood and <br /> Coal Basin mine entries . <br /> Faulting is a significant determ=rant of the <br /> drainage pattern in the Coal Basin , having directly or <br /> 5 <br />