Of those mentioned in the above list, carbon and sulphur are the only ones ever found in the elementary condition in clays. The others are usually found in combination with each other. Thus, for example, silicon unites with oxygen to form the compound known as silica, which consists of one atom of silicon and two atoms of oxygen, and which would be designated by the symbol SiO2. Similarly, two of aluminum will unite with three of oxygen, forming the compound known as alumina and represented by the symbol Al2O3; again, iron in similar combination may give either FeO or Fe2O3; or CaO (lime) may be formed from calcium and oxygen. Carbon and oxygen form CO2, known as carbon dioxide or carbonic acid gas. If the latter unites with CaO, we get a compound expressed by the symbol CaCO3, and called lime carbonate; CaO and SiO2 may unite, giving CaSiO3, which is called a silicate of lime, because it is a compound containing calcium, silicon and oxygen.

The elements are divisible into two groups, the one known as acid elements, the other as basic elements or bases. The latter are commonly oxides of the metallic elements, and include CaO (lime), MgO (Magnesia), Al2O, (alumina), Fe2O, (ferric oxide), K2O (potash), Na2O (soda). The acids and bases are strongly opposed in their characters, and, while there is little or no affinity between members of the same group, those of opposite groups show a marked affinity for each other. An acid, therefore, tends to unite with a base under favorable conditions, these conditions being either the presence of moisture or heat, both of which promote chemical activity and combination. Compounds formed by the union of acid elements and basic elements are termed salts, and the different ones possess a different degree of permanence or destructibility. Thus, some exist only at low temperatures, and are broken up or pass off in gaseous form at a red heat, while others may form only at a temperature of redness or higher.

oceans, and those portions of the original rocks which projected above the ocean were attacked by the weather, in the same manner as described on pages 4-6, the products of rock decay being washed down into the seas, where they were deposited as sediments. The elements of the original rocks would, therefore, be found partly in these sediments and partly in solution in the sea water.

Clays contain a great many different chemical compounds, of more or less definite chemical composition, and often having a definite form. Each of these represents a mineral species, possessing definite physical characters, which could be easily seen if the grains of clay were large enough. The latter is the case, however, with only a few of the scattered, coarse grains, which a clay may contain, and, consequently, it is necessary to use a microscope in order to identify the various mineral grains present in any clay, as even a powerful hand glass cannot ordinarily distinguish them.

MINERALS FOUND IN CLAY.

The number of different minerals present in a clay is often large and depends partly on the mineralogical composition of the rock or rocks from which the clay has been derived, and partly on the extent to which the mineral grains in the clay have been destroyed by weathering. As the result of this weathering action, new minerals are sometimes formed, and we can thus recognize two groups of minerals in the clay, viz., primary and secondary constituents. Those which are probably in most cases of primary character include quartz, feldspar, calcite, gypsum, mica, pyrite, dolomite, iron ores, hornblende and rutile. The chief secondary ones are kaolinite, together with closely allied mineral species and limonite, but calcite, quartz,1 gypsum and pyrite may also be of secondary origin.

Quartz. This mineral, which is silica chemically, is found in at least small quantities in nearly every clay, whether residual or sedimentary, but the grains are rarely large enough to be seen with the naked eye. They are translucent or transparent, usually of angular form in residual clays, and rounded in sedimentary ones, on account of the rolling they have received while being washed along the river channel to the sea, or dashed about by the waves on the beach previous to their deposition in deeper still water. Quartz may be colorless, but it is often colored

1 Some writers argue that much of the quartz found in soil is of secondary origin, but this is often difficult to prove.

superficially red or yellow by iron oxide. It breaks with a glassy, shell-like fracture, and is a very hard mineral, being seven in the scale of hardness. It will, therefore, scratch glass, and is much harder than most of the other minerals commonly found in clay, with the exception of feldspar. Quartz at times forms nodules, which have no crystalline structure, and are termed flint. These are sometimes seen in the Pensauken gravels of the State. Quartz pebbles are not at all uncommon in some of the Cape May and Cohansey clays, and most of the sand grains found in the coarse, gritty surface clays are quartz. This mineral also forms most of the hard pebbles found in the "feldspar" beds of the Woodbridge district.

Both quartz and flint are highly refractory, being fusible only at cone 35 of the Seger series (see Fusibility, Chap. IV), but the presence of other minerals in the clay may exert a fluxing action and cause the quartz to soften at a much lower temperature. (See Silica, below, and also The Fire Clays and FireBrick Industry, Chap. XVI.)

Feldspar.-Feldspar is a mineral of rather complex composition, being a mixture of silica and alumina, with either potash, or with lime and soda, and occurring usually in red, pink or white grains. When fresh and undecomposed, the grains have a bright lustre, and split off with flat surfaces or cleavages. Feldspar is slightly softer than quartz, and while the latter, as already mentioned, scratches glass, the former will not. Feldspar rarely occurs in such large grains as quartz, and, furthermore, is not as lasting a mineral, being easily attacked by the weather or soil waters, and so decomposed to a whitish clay. This change can be seen in the so-called "feldspar" deposits of the Woodbridge district, which were originally a mixture of quartz and feldspar pebbles; the latter, however, have been mostly changed to kaolinite, and it is possible to find pieces showing all stages in the change from feldspar to kaolin in the same bank.

'The hardness of minerals is expressed in terms of Mohs' scale, which is made up of ten minerals, No. 1 being the softest and No. 10 the hardest. The series includes the following minerals: 1, Talc; 2, Gypsum; 3, Calcite; 4, Fluorite; 5, Apatite; 6, Orthoclase; 7, Quartz; 8, Topaz; 9, Corundum; 10, Diamond.

There are several species of feldspar, which vary somewhat in their chemical composition, and are known by different names, as shown below.

Mica. This is one of the few minerals in clay that can be easily detected with the naked eye, for it occurs commonly in the form of thin, scaly particles, whose bright shining surface renders them very conspicuous, even when small. Very few clays are entirely free from mica, even in their washed condition, for, on account of the light scaly character of the mineral, it floats off with the clay particles. The so-called "kaolins" of the Woodbridge district contain much mica, and it is also noticeable in the hollow-ware clays used along the Raritan river, as well as in Clay Marl I. Mica is rarely seen in the Alloway and Cape May clays.

There are several species of mica, all of rather complex composition, but all silicates of alumina, with other bases. Two of the commonest species are the white mica or muscovite, and the black mica or biotite. The former is a silicate of alumina and potash, and the latter a silicate of alumina, iron oxide and magnesia. Of these two, the muscovite is the most abundant in clay, because it is not readily attacked by the weathering agents. The biotite, on the other hand, rusts and decomposes much more rapidly on account of the iron oxide which it contains. effect of mica in burning is mentioned under alkalies.

The

Iron Ores. This title includes a series of iron compounds, which are sometimes grouped under the above heading, because they are the same compounds that serve as ores of iron, when found in a sufficiently concentrated form. The mineral species included under this head are:

1

Limonite, (2Fe:Os,3H2O).

Hematite (Fe2O3).

Magnetite (FeO.).

Siderite (FeCO1).

The first is an oxide, with three parts of water (a hydrous oxide), the second and third are oxides, and the fourth is a carbonate.

Limonite. This has the same composition as iron rust. It occurs in various forms, and is often widely distributed in many clays, its presence being shown by the yellow or brown color of the material. When the clay is uniformly colored, the limonite is evenly distributed through it, sometimes forming a mere film on the surface of the grains; at other times it is, collected into small rusty grains, or again forms concretionary masses of spherical or irregular shape; in still other clays it is found in the form of stringers and crusts, extending through the clay in many directions (Pl. II, Fig. 2). These crusts are common in some portions of Clay Marl II, as at Collingswood, or occasionally also in the Miocene clays. The concretions are abundant in some of the weathered clays at Lorillard, and the beds of sandstone, found in many of the sand and gravel deposits associated with the clays, are caused by limonite cementing the sand grains together.

Hematite, the oxide of iron, is of a red color and may be found in clays, but it changes readily to limonite on exposure to the air and in the presence of moisture.

Magnetite, the magnetic oxide of iron, forms black magnetic grains, and, while not common, is sometimes found when the material is examined microscopically. Like the hematite, it changes to limonite.

Siderite, the carbonate of iron, may occur in clay in the following forms: 1. As concretionary masses of variable size and shape, often strung out in lines parallel with the stratification of the clay. These are more abundant in shales than in clay, and, if near the surface, the siderite concretions change. to limonite on their outer surface. 2. In the form of crystalline grains, scattered through the clay and rarely visible to the naked eye. 3. As a film coating other mineral grains in the clay.