The origin of thick deposits of gypsum without associated deposits of salt and of deposits of salt of great thickness are still open questions and no published hypothesis seems adequate to explain them. This was brought forcibly to the writer’s attention by a study of the gypsum deposits of the Red Beds of Wyoming, some of which are more than 40 feet thick, and the modified bar hypothesis outlined below was gradually evolved. As the various theories have been fully explained in many places, 2 it is not necessary to restate them; but a consideration of the more important phenomena that they do not explain seems worth while.

Phenomena not explained by published Hypotheses

The main difficulties in explaining the origin of thick gypsum deposits are: 1. In accounting for the volume of water required to contain the calcium sulphate in solution, necessitating a depth of basin in excess . . .

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