Abstract
Near the base of the Decorah shale in Minnesota is a thin clay layer that represents an altered pyroclastic deposit, for it consists of montmorillonite retaining a pumiceous texture and contains sanidine, quartz, biotite, apatite, and zircon. It differs materially from the associated sediments, but resembles the Ordovician bentonites of Kentucky and Tennessee with which it has been correlated. The clay of the St. Peter-Platteville transition zone appears to be a normal Ordovician sediment and gives no suggestion of volcanic origin.
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