ington is a sand-stone obtained from quarries on Acquia Creek and the Rappahannock. Much of this stone is feldspathic, possesses but little strength, and disintegrates rapidly. The red sand-stones which are used in our large cities are either from quarries in a formation extending from the Hudson to North Carolina, or from a separate deposit in the Valley of the Connecticut. The most durable and hard portions of these formations occur in the neighborhood of trap dikes. The fine flagging-stone used in our cities is mostly obtained either from the Connecticut quarries, or from others near the Hudson, in the Catskill group of mountains. Many quarries, which yield an excellent building stone, are worked in the extensive formations along the Appalachian range, which extends through the interior, through New York and Virginia, and the intermediate States.

17. Argillaceous Stones. The stones arranged under this head are mostly composed of clay, in a more or less indurated state, and presenting a laminated structure. They vary greatly in strength, and are generally not durable, decomposing in some cases very rapidly, from changes in the metallic sulphurets and salts found in most of them. The uses of this class of stones are restricted to roofing and flagging.

18. Roofing Slate. This well-known stone is obtained from a hard, indurated clay, the surfaces of the lamina having a natural polish. The best kinds split into thin, uniform, light slabs; are free from sulphurets of iron; give a clear ringing sound when struck; and absorb but little water. Much of the roofing slate quarried in the United States is of a very inferior quality, and becomes rotten, or decomposes, after a few years' exposure. The durability of the best European slate is about one hundred years; and it is stated that the material obtained from some of the quarries worked in the United States is not apparently inferior to the best foreign slate brought into our markets. Several quarries of roofing slate are worked in the New England States, New York and Pennsylvania.

19. Graywacke Slate. The composition of this stone is mostly indurated clay. It has a more earthy appearance than argillaceous slate, and is generally distinctly arenaceous. Its colors are usually dark gray, or red. It is quarried principally for flagging-stone.

20. Hornblende Slate. This stone, known also as green stone slate, properly belongs to the silicious class. It con

sists mostly of hornblende having a laminated structure. It is chiefly quarried for flagging-stone.

21. Calcareous Stones. Lime is the principal constituent of this class, the carbonates of which, known as limestone and marble, furnish a large amount of ordinary building stone, most of the ornamental stones, and the chief ingredient in the composition of the cements and mortars used in stone and brick work. Limestone effervesces copiously with acids; its texture is destroyed by a strong heat, which also drives off its carbonic acid and water, converting it into quick lime. By absorbing water, quick-lime is converted into a hydrate, or what is known as slaked lime; considerable heat is evolved during this chemical change, and the stone increases in bulk, and gradually crumbles down into a fine powder.

The limestones present great diversity in their physical properties. Some of them seem as durable as the best silicious stones, and are but little inferior to them in strength and hardness; others decompose rapidly on exposure to the weather; and some kinds are so soft, that when first quarried, they can be scratched with the nail, and broken between the fingers.

The limestones are generally impure carbonates; and we are indebted to these impurities for some of the most beautiful, as well as the most valuable materials used for constructions. Those which are colored by metallic oxides, or by the presence of other minerals, furnish the large number of colored and variegated marbles; while those which contain a certain proportion of clay, or of magnesia, yield, on calcination, those cements which, from their possessing the property of hardening under water, have received the various appellations of hydraulic lime, water lime, Roman cement, etc.

Limestone is divided into two principal classes, granular limestone and compact limestone. Each of these furnishes both the marbles and ordinary building stone. The varieties not susceptible of receiving a polish are sometimes called common limestone.

The granular limestones are generally superior to the compact for building purposes. Those which have the finest grain are the best, both for marbles and ordinary building stone. The coarse-grained varieties are frequently friable, and disintegrate rapidly when exposed to the weather. All the varieties, both of the compact and granular, work freely under the chisel and grit-saw, and may be obtained

in blocks of any suitable dimensions for the heaviest struc

tures.

The durability of limestone is very materially affected by the foreign minerals it may contain; the presence of clay injures the stone, particularly when, as sometimes happens, it runs through the bed in very minute veins: blocks of stone having this imperfection soon separate along these veins on exposure to moisture. The protoxide, the proto-carbonate, and the sulphuret of iron, are also very destructive in their effects; frequently causing, by their chemical changes, rapid disintegration.

Among the varieties of impure carbonates of lime, the magnesian limestones, called dolomites, merit to be particularly noticed. They are regarded in Europe as a superior building material; those being considered the best which are most crystalline, and are composed of nearly equal proportions of the carbonates of lime and magnesia. Some of the quarries of this stone, which have been opened in New York and Massachusetts, have given a different result; the stone obtained from them being, in some cases, extremely friable.

22. Marbles. The term marble is now applied exclusively to any limestones which will receive a polish. Owing to the cost of preparing marble, it is mostly restricted in its uses to ornamental purposes. The marbles present great variety, both in color and appearance, and have generally received some appropriate name descriptive of these accidents.

23. Statuary Marble is of the purest white, finest grain, and free from all foreign minerals. It receives that delicate polish, without glare, which admirably adapts it to the purposes of the sculptor, for whose use it is mostly reserved.

24. Conglomerate Marble. This consists of two varieties; the one termed pudding stone, which is composed of rounded pebbles embedded in compact limestone; the other termed breccia, consisting of angular fragments united in a similar manner. The colors of these marbles are generally variegated, forming a very handsome ornamental material.

25. Bird's-eye Marble. The name of this stone is descriptive of its appearance, which arises from the cross sections of a peculiar fossil (fucoides demissus) contained in the mass, made in sawing or splitting it.

26. Lumachella Marble. This is obtained from a limestone having shells embedded in it, and takes its name from this circumstance.

27. Verd Antique. This is a rare and costly variety, of a beautiful green color, caused by veins and blotches of serpentine diffused through the limestone.

28. The terms veined, golden, Italian, Irish, etc., given to the marbles found in our markets are significant of their appearance, or of the localities from which they are procured.

29. Limestone is so extensively diffused throughout the United States, and quarried, either for building stone or to furnish lime, in so many localities, that it would be impracticable to enumerate all within any moderate compass. One of the most remarkable formations of this stone extends, in an uninterrupted bed, from Canada, through the States of Vermont, Mass., Conn., New York, New Jersey, Penn., and Virg., and in all probability much farther south.

Marbles are quarried in various localities in the United States. Among the most noted are the quarries in Berkshire Co., Mass., which furnish both pure and variegated marbles; those on the Potomac, from which the columns of conglomerate marbles were obtained that are seen in the interior of the Capitol at Washington; several in New York, which furnish white, the bird's-eye, and other variegated kinds; and some in Conn., which, among other varieties, furnish a verd antique of handsome quality.

Limestone is burned, either for building or agricultural purposes, in almost every locality where deposits of the stone occur. Thomaston, in Maine, has supplied for some years most of the markets on the sea-board with a material which is considered as a superior article for ordinary building purposes. One of the greatest additions to the building. resources of our country was made in the discovery of the hydraulic or water limestones of New York. The preparation of this material, so indispensable for all hydraulic works and heavy structures of stone, is carried on extensively at Rondout, on the Delaware and Hudson canal, in Madison Co., and is sent to every part of the United States, being in great demand for all the public works carried on under the superintendence of our civil and military engineers. A not less valuable addition to our building materials has been made by Prof. W. B. Rogers, who, a few years since, directed the attention of engineers to the dolomites, for their good hydraulic properties. From experiments made by Vicat, in France, who first there observed the same properties in the dolomite, and from those in our country, it appears highly probable that the magnesian limestones, containing a certain proportion of magnesia, will be found fully equal to

the argillaceous, from which hydraulic lime has hitherto been solely obtained.

Both of these limestones belong to very extensive formations. The hydraulic limestones of New York occur in a deposit called the Water-lime Group, in the Geological Survey of New York corresponding to formation VI. of Prof. H. B. Rogers' arrangement of the rocks of Penn. This formation is co-extensive with the Helderberg Range as it crosses New York; it is exposed in many of the valleys of Penn. and Vir., west of the Great Valley. It may be sought for just below or not far beneath the Oriskany sand-stones of the New York Survey, which correspond to formation VII. of Rogers. This sand-stone is easily recognized, being of a yellowish white color, granular texture, with large cavities left by decayed shells. The limestone is usually an earthy drab-colored rock, sometimes a greenish blue, which does not slake after being burned.

The hydraulic magnesian limestones belong to the formations II. and VI. of Rogers; the first of these is the same as the Black River or Mohawk limestone of the New York Survey. It is the oldest fossiliferous limestone in the United States, and occurs throughout the whole bed, associated with the slates which occupy formation III. of Rogers, and are called the Hudson River Group in the New York Survey. This extensive bed lies in the great Appalachian Valley, known as the Valley of Lake Champlain, Valley of the Hudson, as far as the Highlands, Cumberland Valley, Valley of Virginia, and Valley of East Tennessee. The same stone is found in the deposits of some of the western valleys of the mountain region of Penn. and Virginia.

Thus far no deposits of hydraulic limestones have been found on the Pacific Coast.

The importance of hydraulic lime to the security of structures exposed to constant moisture renders a knowledge of the geological positions of those limestones from which it can be obtained an object of great interest. From the results of the various geological surveys made in the United States and in Europe, limestone, possessing hydraulic properties when calcined, may be looked for among those beds which are found in connection with the shales, or other argillaceous deposits. The celebrated Roman or Parker's cement, of England, which, from its prompt induration in water, has become an important article of commerce, is manufactured from nodules of a concretionary argillaceous limestone, called septaria, from being traversed by veins of sparry carbonate