On the basis of analyses of different types of sands from various localities in order to establish criteria for distinguishing sands of marine and fluviatile origin, the general rule is formulated that, if the frequency percentage of grains with a diameter of three millimeters is greater than 30 percent, the sands are unquestionably marine; if the percentage is between 20 and 30 percent, the sands are probably marine. Application of the rule to a study of red sands of the Nantes region, France, demonstrates that the sands are Pliocene marine deposits formed in place, or (less probably) they are composed of Eocene marine material which was reworked in the Pliocene.

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