MARYLAND GEOLOGICAL SURVEY 179

Shenandoah valley, a part of the Great Valley above described, is com-
posed of a series of blue and gray limestones and dolomites in which
locally slates and sandy shales become imbedded. In certain places
in eastern Washington county beds of pure fine-grained white marble
are also found. The thickness of the Shenandoah formation is esti-
mated to reach approximately 2500 feet. The slatey limestones and
sandy shales are considered to form a series about 1000 feet below the
top of the formation, and the white marble is known to lie below them.
The structure is so complicated that the position of the various mem-
bers of the formation is much obscured, and both the relations of the
beds and the thickness of the formation can only be approximately
given. Fossils are found in the lower portion of the limestones, but
they are exceedingly rare. They are mainly trilobites and brachio-
pods of Cambrian age. The upper layers of the formation contain an
abundance of fossils of lower Silurian age, and as no physical break
occurs within the series of deposits the line between the Cambrian
and Silurian cannot be definitely determined.

The limestone deposits have been but little altered, but the shaley
beds have been generally more metamorphosed with the production
of mica, which causes a more or less clearly defined schistosity. The
decay of the limestone through solution has left an insoluble residuum
of red clay, through which protrude at times beds of harder materials.
The more rapid solution of the Shenandoah limestone than the rocks
of the other formations has produced the broad fertile Hagerstown
valley. Similar deposits also underlie much of the Frederick valley
as well.

THE SILURIAN PERIOD.

The rocks of the Silurian period are found to the west of the Cam-
brian formations, which have just been described. They constitute
a portion of the Great Valley, and together with the Devonian de-
posits enter into the formation of the Appalachian Mountains proper.
They consist of sedimentary materials that have been but moderately
metamorphosed since they were deposited, although at times sub-
jected to considerable structural disturbances. Six divisions have been
recognized in the sequence of Silurian deposits, known respectively