198 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

some points the beds are carbonaceous, as at the mouth of the Patuxent
river, where a bed of lignite several feet in thickness is exposed at
water level.

The fossils are very numerous, consisting mainly of diatoms, corals,
mollusks, brachiopods and cetaceans, which admit of a division of the
strata into faunal zones, as in the case of the Eocene. Three such
faunal zones have been described in the Miocene of Maryland, viz.:
1. The Plum Point fauna, 2. The Jones' Wharf fauna, 3. The St.
Mary's fauna, the determination in each case being based entirely
upon the molluscan types.

THE LAFAYETTE FORMATION. —The Lafayette formation, so called
on account of the similarity of the strata in Maryland to those
described by Hilgard in Mississippi under that name, widely covers
the deposits of the Coastal Plain, hitherto described, and occupies a
broad area throughout the southern and eastern portions of the state.
In southern Maryland the Lafayette formation covers the higher
levels, but is found at a much less elevation in the eastern counties,
where it passes eastward beneath the covering of Pleistocene deposits.

The deposits of the Lafayette formation consist of gravels, sands
and clays, which are very irregularly stratified and often change
rapidly within narrow limits. Toward the ancient shore line the for-
mation is a coarse gravel, through which is scattered a yellowish
sandy loam, the whole cemented at times by hydrous iron oxide into
a more or less compact conglomerate. The eastward extension of the
formation shows a gradual lessening of the coarser elements and a
larger admixture of loam. The constituent materials put of which
the deposits are formed are frequently much weathered and become
a pronounced arkose. The deep orange color of the strata is highly
characteristic of the formation. The deposits seldom exceed 25 feet
in thickness, although at some points a thickness considerably greater
has been observed.

The Lafayette formation has afforded no fossils in the state of
Maryland to indicate its geological age. From the fact, however, that
the strata rest unconformably upon the underlying Chesapeake de-
posits of Miocene age and are in turn unconformably overlain by