The Lower Cretaceous formations afford important brick clays, the Raritan, however, furnishing chiefly buff-burning clays, although redburning ones occur. The clays from this formation are best developed in Anne Arundel County. The Patapsco formation which underlies the Raritan contains a large amount of highly variegated clays and extends. in a broad belt across the State near the western margin of the Coastal Plain. The clays of the Patapsco formation are more plastic than the Columbia clays and as a general rule occur in beds of much greater thickness. They are particularly well adapted to the manufacture of stiffmud brick while the Columbia clays are rather too gritty for this purpose. Next to the Columbia clays they are the most important brick clays in the eastern section of the State. They are located for the most part near the head of tide along the leading railroad lines and therefore possess great commercial importance.

In the Jurassic the Arundel formation affords large supplies of ironore clays which are well adapted to the making not only of common brick but also of pressed brick. They are moderately silicious, highly plastic, and have sufficient iron to burn to a good red color. At some localities the Arundel clays are comparatively free from iron so they burn buff instead of red and lend themselves well to the production of terra cotta and roofing tile.

The residual clays of the Piedmont region are derived from either gneisses, granites, limestones, or schists, as a general rule, and in almost every case are quite ferruginous so that they not only burn to a deep red product but may do so at a comparatively low temperature. Those which are derived from a basic igneous rock, such as gabbro or peridotite, usually have a very high plasticity and consequently show a high shrinkage in burning. Owing to their high plasticity, however, they generally permit of the admixture of considerable sand, although the manufacturer often has some difficulty in thoroughly incorporating the material with the clay. The residual clays are likely to be variable in their depths owing to the uneven surface of the underlying rock, and consequently they may vary anywhere from 3 or 4 to 25 or more feet in thickness.

The shales suitable for brick making are to be found either in the Devonian or Carboniferous, although up to the present time only the former have been used. With an increase in demand for bricks in the counties of the Appalachian region the Carboniferous shales will no doubt spring into prominence and be opened at a number of points. Many of these shale deposits will also probably be found available for the manufacture of vitrified brick.

TERRA COTTA CLAYS.

The terra cotta industry of Maryland has been comparatively little developed although what has been done is sufficient to show that suitable materials for the purpose are not lacking within the limits of the State. The kinds of materials which have been chiefly employed for this purpose are the buff-burning Arundel clays, the sandy Patuxent clays, and the variegated Patuxent clays. All of these clays are well developed to the south of Baltimore, especially in Anne Arundel and Prince George's counties. At times the variegated Patapsco clay is also well adapted for terra cotta work, particularly in the southern part of Baltimore City.

SEWER-PIPE CLAYS.

The only sewer-pipe clays employed at the present time come from the Arundel formation although it is probable that equally good clays could be obtained from the Pleistocene deposits and from the Patapsco formation. The small importance of the sewer-pipe industry in Maryland at the present time is due rather to trade conditions than to lack of clays. for both the Arundel and Patapsco formations yield materials of considerable plasticity.

FIRE CLAYS.

The refractory clays found in Maryland are obtained either from the Coastal Plain formations or from the Carboniferous deposits of the Appalachian region. The Carboniferous fire clays of Maryland have

long been well known, the deposits having been worked since 1841. The Pottsville formation is the chief source of the clays which are worked at the present time, chiefly along the eastern flank of Savage Mountain. These Carboniferous fire clays occur in two forms, known as the plastic clay or shale, and the flint clay. Both of them are highly refractory in character.

In the Coastal Plain region fire clays are obtained from the Patapsco, Raritan, and Patuxent formations as well as in some instances from the decayed crystalline rocks beneath. The first three of these formations contain lenses or extensive beds of white to yellow-white clays which frequently show a high resistance to fire and can be heated up to the fusing point of cone 27 without in many cases becoming vitrified.

The refractory-ware industry of Maryland is one of the most important branches of the clay-working industry found in the State. Among the more important products are fire-brick, enameled-brick, retorts, stovebrick, and stove-linings. The fire-brick are made both in western Allegany County and in Baltimore. The manufacture of enameled-brick is confined, however, to the former locality. Retorts are made in Baltimore while stove-brick and stove-linings are largely manufactured in Cecil County.

POTTERY CLAYS.

The pottery clays include materials showing a wide range of composition. The clays suitable for the manufacture of stoneware are to be found at many points in the Patapsco formation, especially in Cecil County. At the base of the Patapsco formation in the same county there is often a bed of bluish-gray, very plastic stoneware clay. Aside from these Cecil County stoneware clays the most important are those outcropping along the shore of the Chesapeake from Bodkin Point southward.

Clays suitable for the manufacture of yellow-ware are to be found at a number of points in the Arundel formation and also in the Columbia, both of these formations being extensively drawn upon by the yellow and Rockingham ware manufacturers of Baltimore.

Clays for the manufacture of the common red earthenware are abundant and are obtained from the Columbia, Arundel, and Patapsco formations of the Coastal Plain and from the residual clays of the Piedmont Plateau northeast of Catonsville, and also from the residual clays of the Appalachian region around Hagerstown. The Potomac clays near Baltimore also afford the basis for the manufacture of the higher grades of pottery.

The clay industry is already an important one in Maryland, the average value of the output in recent years being $1,435,000.

THE PORCELAIN MATERIALS.

Maryland, as a State, is well provided with porcelain materials such as flint, vein quartz, feldspar, and kaolin. These are chiefly developed in central Maryland and mined in this region only. They are abundantly present over wide areas but only occasionally are they sufficiently free from colored minerals and coloring matter to meet the requirements of porcelain manufacture.

FLINT.

The flint or quartz is derived from unsually large and pure masses of vein quartz or from portions of the gneiss and granite along their contact where the original rocks have been reduced to white pulverulent quartz. Flint has been sought most successfully in Cecil, Harford, and Baltimore counties. Most of the material occurs as veins intersecting the country rock, generally gabbro, serpentine, or granite, as in the vicinity of Castleton and along Deer Creek in Harford County. Here and elsewhere where the vein quartz is utilized it is necessary to grind the material to a flour and to facilitate this grinding it is customary to roast the blocks of quartz and then cool them suddenly by pouring on water, as is done at the flint works at Conowingo, Cecil County. The flint from the granite-gneiss contacts require no roasting but need to be ground to a flour to meet requirements. The flint flour is shipped in bags to different points within and without the State, chiefly Trenton, New Jersey, where it is employed in the manufacture of porcelain,