Siliceous filling of epidermal cells of grasses (Gramineae) produces particles known to soil microscopists as opal phytoliths. Reports of phytoliths from rocks are rare. Jones studied samples from the Tertiary Flagstaff and North Horn Formations in Utah, the Green River Formation in Wyoming, the Truckee Diatomite in Nevada, and the Ogallala Group in South Dakota. Phytoliths were found in all but the Green River and Truckee, being especially common near the base of the Flagstaff and in several beds of the Ogallala. This survey suggests that phytoliths can be useful as indicators of grass vegetation in Cenozoic rocks. Opal reactions may also provide data on diagenetic history of the rocks.

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