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1. A Seam - This is a thin coal of variable thickness (three inches to • several feet). It generally marks the top of the Rollins Sandstone. 2. Lower B Seam - A mineable attritial coal with a characteristic thickness of six to seven feet. It sometimes carries thin, coaly shale splits. 3. Upper B Seam - Separated from the Lower B Seam by generally six to ten feet of coaly shale, the Upper B Seam is consistently the thickest coal of the Mesa Verde sequence in this area. In the vicinity of the Hawk's Nest Mine, it has a characteristic thickness of thirteen to fourteen feet of attritial coal. In some instances it carries thin splits of coaly shale. 4. C Seam - This is the uppermost coal of the Bowie Member. It is generally a clean attritial coal with a characteristic thickness of six to seven feet. The clastic sediments associated with these coals are generally fine- grained and carbonaceous. They range from shales and siltstones to fine- grained sandstones. Generally they are intimately interbedded, often showing considerable boring activity, and in some cases, bioturbation. A few massive • sands are of marine origin, having significant amounts of glauconite. A few shales are fossiliferous, carrying pelecypods identified as Corbula. Thin, light brown mudstone interbeds and, rarely, limestones up to three inches in thickness occur. Environmentally, this sequence apparently represents the back barrier, lower delta plain and transitional lower delta plain. As noted earlier, the mineable coal beds of this member exhibit a consistent thickness. The top part of the Bowie Member is a barren clastic sequence of shale, siltstone, and sand of approximately 175 feet. The Paonia, or Upper Coal Member of the Mesa Verde, contains three and occasionally four mineable coals in a total sedimentary thickness of 400 to 600 feet. These coals are more variable in thickness and lateral extent than the coals of the Bowie Member and apparently represent a sedimentary • 27