The Mowry Shale is a hard, highly siliceous black shale which is stratigraphically the highest Lower Cretaceous formation in Wyoming. W. W. Rubey has suggested that the Mowry Shale originated as an altered volcanic ash, basing his ideas on a similarity between chemical analyses of the Mowry and analyses of bentonite. More recent analyses indicate that this similarity does not exist, and that the Mowry is the result of normal marine sedimentation. The presence of siliceous radiolarian tests and the absence of volcanic glass shards suggests that excess silica in the Mowry was derived from organisms and not from ash.

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