TERTIARY FORMATIONS, 199

alone could be separated with little difficulty from all the other forma-
tions of the Coastal Plain. The deposits consist mainly of a dark bluish-
green, medium fine, argillaceous sand that contains many imprints and
casts of small-sized Mollusca. The sand on exposure to the atmosphere
changes in color, due to oxidation of the iron present, and assumes a
ferruginous brown to yellow color. In certain places the iron has been
almost entirely removed and the sand is gray to white in color. Nor-
mally the St. Marys sand is loose, though locally it may be indurated
by a ferruginous cement. Marl beds occur in numerous places in the
St. Marys, and these strata have been extensively worked in many places
for fertilizing purposes. In certain localities shells are so abundant that
they constitute the greater portion of the beds, but in other places the
sandy matrix in which the shells are found makes up more than half
the deposit. The fossils are usually rather fragile and somewhat rotten,
so that they go to pieces easily and thus make very good material for
the soils. During slavery days the marl deposits were worked exten-
sively and were considered of importance, but within recent years there
has been comparatively little marl dug. The reason for this is the
seareity of labor and the introduction of artificial fertilizers which act
much more quickly and contain certain ingredients not found in the
shell marl. The sand of St. Marys is rather fine in almost all places,
though coarse sand and occasional pebbles are found near the base and
_ along the western limits of the formation, as shown by the sections in
the vicinity of Halifax, given on a later page. A pebble band is not
infrequent near the base of the formation, and sometimes solitary peb-
bles varying up to 1 inch in diameter are found irregularly distributed
through the sand some distance from the base. The coarse materials
have, in almost every case, been derived from the crystalline rocks of
the near vicinity, and some of them show little wearing by water.
_ Black phosphatic pebbles and water-worn bones and sharks’ teeth were
_ probably derived from the Cretaceous or Eocene. Beside the sands and _
‘marls, there are occasional layers of sandy elay and occasional plastic
clay beds interstratified with the sand.
At only one point thus far found within the State does there seem to
be any evidence of diatomaceous material. This occurrence is about one
mile northeast of ‘Wrendale at the foot of a steep bank along Swift |
Creek. The diatomaceous earth there is decidedly sandy and grades
downward into a stratum of sand, :
a he lithologie characteristics of the St. Marys formation are well
exemplified in the numerous sections of this formation given on later