178 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

tion. The shales are of a dull bluish gray color when fresh, and
weather to a light greenish gray. Argillaceous materials predomi-
nate, with frequent small grains of quartz and feldspar, while other
materials derived from the Algonkian volcanics appear sparingly.
The thickness of the Harpers formation is difficult to determine, owing
to the absence of any complete section of it. Its outcrops are every-
where included between faults which have cut off intermediate thick-
nesses. It has been estimated, as the result of a number of measure-
ments, to have a probable thickness of 1200 feet.

The shales have been subjected everywhere to considerable altera-
tion, the feldspathic materials being partially recrystallized into quartz
and mica, with the development of schistosity. The metamorphism
is much more pronounced along the eastern border, in the Catoctin
area, where the change has proceeded so far as to produce a mica-
schist in which small quartz lenses are developed between the layers.
Decomposition has affected the shale to considerable depths, the argil-
laceous materials furnishing a sufficient amount of clay to produce a
soil of some value, but on steep slopes it is easily washed.

THE ANTIETAM FORMATION. —The Antietam formation receives
its name from Antietam creek, along the tributaries of which the
deposits of this formation are most typically developed. The rock is a
sandstone which grades below by gradual transitions into the Harpers
shale. The sandstone is composed of small grains of white quartzite
well worn and sorted, and it contains a small percentage of carbonate
of lime. Its color is almost invariably of a dull brown. It is more
fossiliferous than the other Cambrian formations, remains of trilobites
being not uncommon. The formation has a thickness of about 500
feet.

The Antietam sandstone shows little alteration in its typical area,
but east of Catoctin Mountain there are some very silicious schists
that may possibly represent it. The more calcareous varieties weather
readily, but numerous blocks of the sandstone generally strew the
surface.

THE SHENANDOAH FORMATION (lower part). —The Shenandoah
formation, so called from the fact that it forms the floor of the