way from Harford County to the border of the District of Columbia. The clays are highly carbonaceous, lignitized trunks of trees being often encountered in an upright position with their larger roots still intact. Scattered through the tough dark clays are vast quantities of nodules of iron carbonate, at times reaching many tons in weight, and known to the miners under the name of "white ore." In the upper portion of the formation the carbonate ores have changed to hydrous oxides of iron, which the miners recognize under the name of "brown ore." The largest lenses have been found to reach a thickness of nearly 125 feet. THE PATAPSCO FORMATION.-The Patapsco formation, so called from its typical occurrence in the valley of the Patapsco River, forms the uppermost division of the Potomac group. It extends entirely across the state from the Delaware border to the Potomac River, and throughout this distance is one of the most important members of the Cretaceous series. Beyond Maryland it extends southward to central Virginia. The deposits of this division consist chiefly of highly colored and variegated clays which grade over into lighter colored sandy clays, while sandy bands of coarser materials are at times interstratified. The sands frequently contain much decomposed feldspar and rounded lumps of clay also occur. The sands are often cross-bedded, and all the deposits give evidence of shallow-water origin. The formation is estimated to reach a thickness of 200 feet. The deposits rest unconformably upon the Arundel below and dip from 35 to 40 feet in the mile to the southeast. The Upper Cretaceous The formations referred to the Upper Cretaceous overlie the Lower Cretaceous deposits unconformably. The lowest formation is of estuarine origin in Maryland, while the highest formations are distinctly marine. They apparently represent the Cenomanian and Senonian with possibly the lower portions of the Danian of Europe. THE RARITAN FORMATION.-The Raritan formation, so called from its typical development in the valley of the Raritan River in New Jersey, extends across that state into Delaware and Maryland. It is found in Cecil and Kent counties and extends thence southwestward along the eastern border of Harford and Baltimore counties into Anne Arundel County, where it broadens out and occupies a considerable extent of country along the Severn River. Beyond the Patuxent Valley the area of outcrop narrows, as the result of the transgression of the overlying Upper Cretaceous strata. The deposits of the Raritan formation consist chiefly of thick-bedded and light-colored sands, which at times become gravels. Frequently in the lower portion of the formation the sands grade over into the clays, which are generally light in color and highly siliceous, although they are sometimes deeply colored. The thickness of the Raritan formation reaches about 200 feet. The deposits overlie unconformably the Patapsco sediments below and dip about 35 feet in the mile to the southeast. THE MAGOTHY FORMATION.-The Magothy formation, so called from the Magothy River in Anne Arundel County, overlies the deposits of the Raritan unconformably. It extends as a narrow belt from New Jersey southward along the eastern margin of the earlier formations. The Magothy formation crosses Delaware, Cecil, and Kent counties on the Eastern Shore and Anne Arundel and Prince George's counties on the Western Shore. The materials consist of sands and clays which change rapidly both horizontally and vertically. The sands are commonly of light color, although lenses with bands of darker sands occur. The clays often appear finely laminated with sand layers between and are occasionally nearly black in color, due to the presence of vegetable matter. The thickness of the Magothy formation in Maryland is very variable, ranging from 90 feet down to 20 feet or less. Its average thickness is probably about 50 feet. The formation has a dip of 30 to 35 feet in the mile to the southeast. THE MATAWAN FORMATION.-The Matawan formation receives its name from Matawan Creek, New Jersey, in the vicinity of which it is extensively developed. It lies along the eastern margin of the Magothy formation upon which it rests unconformably. The Matawan formation forms a narrow belt which crosses Delaware, southern Cecil, and northern Kent counties, and then reappears upon the Western Shore in Charles and eastern Anne Arundel counties and thence continues southwestward with constantly narrowing confines across Prince George's County. The deposits of the Matawan formation consist mainly of dark-colored micaceous sandy clays which at times are somewhat more sandy in the upper portions and more argillaceous in the lower portions, although in general the formation is very homogeneous throughout, from Kent County southward. The formation has an average thickness of about 50 feet on the Eastern Shore, but gradually thins southward until it is not over 10 feet in thickness in Prince George's County. The deposits have a dip of from 20 to 30 feet in the mile to the southeast. THE MONMOUTH FORMATION.-The Monmouth formation, so called from its typical development in Monmouth County, New Jersey, overlies the Matawan formation conformably and extends from New Jersey southward across Delaware into Maryland, but is very much less extensively developed in the State of Maryland than to the northward, although some of its chief characteristics still prevail. The Monmouth formation lies to the east of the Matawan deposits already described and forms a narrow belt crossing Delaware, Cecil, Kent, Anne Arundel, and Prince George's counties. THE RANCOCAS FORMATION.-The Rancocas formation, so called from its typical occurrence in the valley of Rancocas Creek in southern New Jersey, where it conformably overlies the Monmouth formation, is well developed throughout that state and in Delaware, but due to the transgression of the basal Eocene deposits is probably lacking in Maryland. In Delaware the Rancocas formation consists of greensand marls which are frequently highly calcareous. The deposits are in general quite arenaceous and on the whole less glauconitic than the Sewell marls in New Jersey, to which horizon they evidently belong. The formation has a thickness of about 20 feet in central Delaware, but gradually thins out toward the Maryland line where the Eocene deposits have transgressed the Rancocas and directly overlie the Monmouth formation. THE TERTIARY PERIOD The Tertiary deposits form part of a complex series of formations that extend from New Jersey southward to the Gulf. At no point in the middle Atlantic region is the series more complete or better exposed than in the Chesapeake Bay district and the bluffs along the Maryland and Virginia streams have been classic ground for the study of American Tertiary strata. These Tertiary beds unconformably overlie the Cretaceous deposits which they gradually transgress landward. The Tertiary of Maryland and Delaware is chiefly represented by the Eocene and Miocene, although deposits of presumably Pliocene age also occur. The Eocene The deposits of Eocene age lie above and to the east of those previously described. They strike across the area from northeast to southwest and can be traced southward into the State of Virginia. In the Potomac Valley they dip about 12 feet in the mile to the southeast. The Pamunkey Group The Pamunkey group, so called from the Pamunkey River in Virginia, has an extensive development both in Maryland and Virginia. The surface of the Pamunkey group is largely covered by deposits of later date. Numerous outcrops occur along the streams, particularly in the valley of the Potomac River, the interstream portions of the country being generally covered by later deposits. The Pamunkey group has been divided into the Aquia and Nanjemoy formations. THE AQUIA FORMATION.-The Aquia formation, so called from Aquia Creek, Virginia, is found unconformably overlying the Cretaceous deposits from Delaware southwestward and southward as far as southern Virginia. From Cecil County it crosses Kent and the northern portion of Queen Anne's County on the Eastern Shore, and thence extends across Anne Arundel, Prince George's, and Charles counties on the Western Shore, being particularly well exposed in the valley of the Potomac. The deposits which consist chiefly of greensands and greensand marls, at times highly calcareous and less frequently argillaceous, have a thickness of about 100 feet at the point where the beds disappear below tide. THE NANJEMOY FORMATION.-The Nanjemoy formation, so called. from Nanjemoy Creek, Charles County, is found conformably overlying the Aquia formation. So far as known it is confined to the Western Shore, outcropping at various points across southern Anne Arundel, northern Calvert, southern Prince George's, and central Charles counties. The most extensive sections of the Nanjemoy formation in Maryland are in the vicinity of Upper Marlboro, along the Patuxent River, and along the Potomac River in southern Charles County from Popes Creek northward. The best part of this section is opposite Charles County along the Virginia bank of the Potomac to the east of Potomac Creek. The deposits consist of greensands, often highly argillaceous and less frequently calcareous than the lower beds, and with here and there layers containing abundant crystals and crystallized masses of gypsum. The thickness of the deposits is about 125 feet where best exposed, although the beds thicken to some extent eastward. The Miocene The Miocene deposits form part of a broad belt of middle Tertiary formations. The strata attain considerable thickness and constitute the most important element in the Coastal Plain series with the possible exception of the Cretaceous formations. The deposits are mainly if not wholly marine and fossils are numerous at most horizons. They constitute a single group known as the Chesapeake group. The Chesapeake Group The Chesapeake group, so called from the characteristic development of the deposits in the Chesapeake Bay region, occupies a wide area of distribution throughout the eastern and southern counties of Delaware and Maryland. It overlies the Eocene formations unconformably and in places along the western margin transgresses them to the Cretaceous deposits below. The surface of the Chesapeake group is for the most part covered by the deposits of later date. Fine outcrops, however, occur along the larger stream channels and in the bluffs bordering Chesapeake Bay and |