are also frequently cross-bedded and are very irregular both in their extent and sequence.

THE ALLEGHENY FORMATION is named from its typical exposure upon the Allegheny River, Pennsylvania. It occupies the basal portion of the basins within the synclines which are outlined by the Pottsville conglomerate in western Allegeny and Garrett counties. It consists of a series of sandstones, shales, limestones, and coal seams.

THE CONEMAUGH FORMATION is approximately the same as the division formerly known as the Lower Barren Coal Measures. It receives its name from its exposure along the Conemaugh River in western Pennsylvania. It overlies the Allegheny formation, with which it is associated at the localities cited in the discussion of that formation. It consists of a series of sandstones, shales, conglomerates, limestones, and coal seams.

THE MONONGAHELA FORMATION (Elkgarden) is approximately the same as the division formerly called the Upper Productive Coal Measures. It is named from its typical exposure along the Monongahela River in Pennsylvania. In Maryland this formation is restricted to the Georges Creek-Potomac basin. It consists of a series of shales, sandstones, limestones, and coal seams.

THE PERMIAN PERIOD

The rocks which are referred to the Permian are confined to the central portion of Georges Creek Valley in western Allegany County, where they rest with apparent conformity upon the Carboniferous deposits below. The single formation recognized in these rocks is denominated the Dunkard formation.

THE DUNKARD FORMATION is approximately the same as the division formerly known as the Upper Barren Coal Measures. It is named from its exposure on Dunkard Creek, Pennsylvania. Its strata apparently conformably overlie the Monogahela formation of Carboniferous age. It occurs in patches along the center of the Georges Creek Valley where erosion has left fragments capping the top of the higher lands. It consists of limestones, sandstones, shales, and coal seams.

IGNEOUS ROCKS

The igneous rocks of the Appalachian district are limited to the eastern division of the Blue Ridge and Catoctin mountains. No rocks of igneous origin have been found in the part of Maryland lying west of the Hagerstown Valley. The igneous rocks of the Blue Ridge-Catoctin area are similar to those already described and are classified under the heads of Acid volcanics, Basic volcanics, and Granites.

The Acid Volcanics

The acid volcanics of the Appalachian district of Maryland occupy an irregular area north and northeast of Myersville near the head of the Middletown Valley between the Blue Ridge and Catoctin mountains. They form the higher slopes of the headwaters of Catoctin Creek and extend well up to the state line. They are closely related to similar masses in Pennsylvania and Virginia and are represented in several smaller outlying masses, some of which have already been described.

The Basic Volcanics

The basic volcanics of the Maryland Appalachian district are more wide-spread than the acid volcanics, occupying between two and three times as much surface as the latter. Like them they are also represented in masses of similar rock to the north and south of Maryland, and in detached bodies to the east of Catoctin Mountain, as already described. They were formed by intrusions of basic material both before and after the formation of the acid volcanics. The products of these intrusions, which were originally quite similar, have been changed by the varying conditions to which they have been subjected since they were first formed. The present rocks have been classed as "Andesite" and "Catoctin. schists." The andesite is found in adjacent areas in Virginia but has not been recognized in Maryland where the sole representative of the basic. volcanics is the Catoctin schist. This schist forms practically all of the region between the eastern flanks of the Blue Ridge and the western flanks. of Catoctin Mountain, except the central area occupied by acid volcanics.

and the southwestern part of the Middletown Valley along the Potomac, where the volcanic rocks seem almost crowded out by the numerous intrusions of granite.

The Granites

Intimately intermingled with and cutting the acid and basic volcanics already described are intricately anastamosing bodies of granite which occur in long, narrow belts varying in breadth from 1 yard to 6 miles, with an average width of perhaps 100 yards. By far the greatest development is in the valley lands north of the Potomac River in the Middletown Valley.

THE COASTAL PLAIN

The area of low land and shallow sea floor which borders the Piedmont Plateau on the east and passes with constantly decreasing elevation eastward to the margin of the continental shelf has been described under the name of the Coastal Plain. It is made up of geological formations of late Mesozoic and Cenozoic age. These later formations stand in marked contrast to the older strata to the westward in that they have been but slightly changed since they were deposited. Laid down one above another upon the eastern flank of the Piedmont Plateau when the sea occupied the present area of the Coastal Plain, these later beds form a series of thin sheets that are inclined at low angles seaward so that successively later formations are encountered in passing from the inland border of the region toward the coast. Oscillation of the sea floor with some variation both in the angle and direction of tilting took place, however, during the period of Coastal Plain deposition. As a result the stratigraphic relations of the formations, which have generally been held to be of the simplest character, possess in reality much complexity along their western margins, and it is not uncommon to find that intermediate members of the series are lacking, as a result of transgression, so that the discrimination of the different horizons, in the absence of fossils, often requires the utmost care.

The Coastal Plain sediments were laid down after a long break in time following the deposition of the red sandstones and shales (Newark forma

tion) of Triassic age, which overlie the crystalline rocks of the western division of the Piedmont Plateau, and complete the sequence of geological formations found represented in Maryland and Delaware. From the time deposition opened in the coastal region, during early Cretaceous time, to the present, constant sedimentation has apparently been going on, notwithstanding the fact that frequent unconformity appears along the landward margins of the different formations.

The formations comprise the following:

The Potomac Group (Lower Cretaceous)

The formations here described include what was long known as the Potomac formation, so called from the Potomac River, in the drainage basin of which the deposits of this age are well shown, but which is now

recognized as representing several quite distinct stratigraphic units. These formations have only been found in their full development in the middle Atlantic coastal area, while the lower formation extends southward to the Alabama area, and the upper formations extend both to the northward and southward. The Potomac was deposited largely, if not entirely, under continental conditions.

The Potomac group is divided into the Patuxent, Arundel, and Patapsco formations of Lower Cretaceous age.

THE PATUXENT FORMATION.-The Patuxent formation, so called from its typical development in the upper valleys of the Little and Big Patuxent rivers, is the basal formation of the Coastal Plain series, and is found lying directly upon the crystalline rocks of the Piedmont Plateau. It appears near the landward margin of the Coastal Plain and has been traced as a narrow and broken belt from Cecil County across Harford, Baltimore, Anne Arundel, and Prince George's counties to the border of the District of Columbia.

The deposits consist mainly of sand, sometimes quite pure and gritty, but generally containing a large amount of kaolinized feldspar, producing a clearly defined arkose. Clay lumps are at times scattered in considerable numbers through the arenaceous beds. Frequently the sands pass over gradually into sandy clays, and these in turn into argillaceous materials, which are commonly of light color, but often become highly colored and are locally not unlike the variegated clays of the Patapsco formation. The more arenaceous deposits are cross-bedded, and the whole formation. gives evidence of shallow-water origin. The dip of the beds is about 40 feet in the mile to the southeast. The Patuxent formation is estimated to attain a thickness of about 350 feet, but it may be considerably thicker at some points.

THE ARUNDEL FORMATION.-The Arundel formation, so called from Anne Arundel County, where the strata are well developed, consists of a series of large and small lenses of iron ore-bearing clays which occupy ancient depressions in the surface of the Patuxent formation and are unconformable to that formation. These lenses have been traced all the