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• units . Coal seams , including the two seams mined by Mid- <br /> Continent , lie above the Rollins and Middle Sandstone units . <br /> The Mesaverde group is customarily described <br /> in terms of a lower formation, called the Iles , and an upper <br /> formation, called the Williams Fork. The Iles Formation <br /> overlies the Mancos Shale and , as noted , the Williams Forma- <br /> tion overlies the Iles . <br /> The Iles Formation consists of two sandstone <br /> members divided by a 1 , 000 ' "tongue" of shale . The lower <br /> member , the Cozzette , is a 6-foot layer of very fine grained <br /> silty sandstone , which overlies a 40-foot thick layer of <br /> silty to sandy shale . <br /> The upper sandstone member of the Iles <br /> • formation, known as the Rollins , is 120 ' to 140 ' thick. A <br /> shale tongue of 5 'to 30 ' thickness , and identical to the <br /> underlieing shale , occurs within the Rollins member 25 ' to <br /> 75 ' above the base . The Rollins Sandstone varies in grain <br /> from very fine to coarse , but medium size predominates . <br /> Calcareous ironstone concretions , some as <br /> large as 20' or 30 ' long , occur mainly below the shale tongue . <br /> The sandstone varies from feldspathic to lithic , and is <br /> cemented by calcite and silica until the top 51to 201 , <br /> where a silica-clay matrix predominates . The silica-clay <br /> matrix is common throughout the western Upper Cretaceous <br /> where coal directly overlies sandstone . <br /> . The Rollins , which is the top of the Iles <br /> Formation, forms the base of the coal series . Drill hole <br /> 8 -- - <br />