INTERPRETATION OF FOSSIL PLANTS 
13 
Where muds accumulate in lagoons cut off from direct wave- 
action, and the vegetation which contributed its debris to the 
deposit grew in the immediate vicinity, as was the case with so 
many of the lagoons that lined the shores of the lower Eocene 
(Wilcox beds) Mississippi embayment, a great variety of nearly 
perfectly preserved fossils will result. Leaves that have accumu¬ 
lated in the back waters of bayous may decay very slowly because 
of the antiseptic qualities of the water. They will tend to ac¬ 
cumulate in matted layers, and very frequently will show traces 
of the galleries of Tinead and other insect larvae. In each sepa¬ 
rate case it becomes a question of the interpretation of various 
lines of evidence, and no general empirical rules can be formu¬ 
lated. For example, the plants found in the Miocene diatomace- 
ous deposits of Virginia are associated with a shallow-water 
marine fauna. From the presence of the diatomaceous beds it 
is inferred that the land was low and the run-off was slow, bring¬ 
ing little beside silt to the basin of sedimentation. The presence 
in these deposits of quantities of the deciduous twigs of the 
cypress (Taxodium), as well as its seeds, corroborates the in¬ 
ferred mature drainage conditions, with the presence of coastal 
bayous and palustrine river-bottoms, and the presence of the 
normal members of such a plant assemblage, some of the leaves 
of which are incrusted with Membranipora, completes the pic¬ 
ture.® 
It is not always possible to distinguish lagoonal from estuarine, 
or delta, deposits by means of the fossil plants. In general the 
last two are more likely to contain species of riverside, or per¬ 
haps upland plants, that have been carried some distances. Per¬ 
haps fruits gathered by a river system will represent species 
whose leaves could not withstand such a journey. This is notably 
the case with the Eocene flora of Sheppey, in the Thames basin, 
whose flora of fruits furnishes a striking contrast to the synchron¬ 
ous flora of leaves of autochthonous species preserved in the pipe 
clays of Alum Bay. Delta deposits in particular, unless they con¬ 
tain permanent bodies of still water, generally lack well pre¬ 
served leaves, but they usually show much carbonaceous accumu¬ 
lations and prostrate logs left in layers by flood-waters, as in the 
Late Cretacic Tuscaloosa formation of Alabama. Palustrine 
5 U. S. Geol. Surv., Prof. Paper 98, 61-73, 1916. 
