A single leaf only is referred to this form. It now grows commonly from Pennsylvania to Florida and west to Indiana and Texas.
The species thus far determined all belong to living species. Some of them enjoy a wide distribution and are still found growing in the region, while others are now only found to the north. One in particular, Potamogeton robbinsii, is confined to the north.
It seems probable from the evidence of the plants that they were pushed down from the north during the ice invasion and were entombed, while the species has retreated again to the colder area.
The occurrence of Potamogeton robbinsii in these beds is of special interest, since it practically demonstrates that there was during Glacial times a movement of water from the edge of the ice near Beaver, Pa., southward along the Mononguhela valley through the escape wiers just described, which brought with it this northern plant. It is possible that a systematic search would bring to light many other such northern forms, as well as throw much light upon the slight changes that have taken place in species since the Glacial epoch, because there can be little doubt that these plants were embedded in their present matrix during the Ice age The particular locality in which the plants occur is near the head waters of two little streams which rise against each other, and then flowing north, empty into the Monongahela, their mouths being two and one- half miles apart.
The broad level summit between the heads of the two streams is covered with the clay deposits up to 251 feet above the present river bed, and at one locality (Mr. Baker's well) they have a thickness of 65 feet. The surrounding hills are made up of the soft shales of the Barren or Elk River coal measures, and it is in just such a sheltered bayou back from the main channel of the river that we would expect to find such deposits in lake Monongahela.
A fine quality of clay for common blue stoneware, or crockery, is always found among these deposits where any considerable stream empties into the Monongahela from the west (soft rock areas), while sand and boulders predominate at the mouths of those streams draining from the east (mountain or sand-rock areas). The celebrated pottery clays of Geneva and Greensboro, Pa., just north from the West Virginia-Pennsylvania line are deposited opposite the mouth of Dunkard creek, a stream entering the Monongahela from the west, and drain-
