372 The American Geologist December, ii-96 
A single leaf only is referred to this forri). 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 re- 
gion, while others are now only found to the north. One in particular, 
Potai)i()(jeton rohbitiKii, is confined to the north. 
It seems probable from the evidence of the plants that they were 
I)ushed down from the north during the ice invasion and were en- 
tombed, while the species has retreated again to the colder area. 
The occurrence of Potainogeton rohbinfii.i in t'les^ 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 Monongahela 
valley through the escape wiers just described, which brought 
with it this northern plant. It is possible that a systematic 
search would bring to liglit many otlier 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 M^ell) 
they have a thickness of 65 feet. The surrounding hills are 
made up of the soft shales of the Barren or P^lk River coal 
measures, and it is in just such a sheltered ba3'"0u 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 crock- 
er}^ is always found among these deposits where any consid- 
erable 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-Pennsyl- 
vania line are deposited opposite the mouth of Dunkard creek, 
a stream entering the Monongahela from the west, and drain- 
