The silica in limestones is often segregated into nodular masses of varying sizes, to make concretions of flint. Such masses have grown in the limestone, and, while growing, have either pushed away, or dissolved the adjacent limestone, so that the flint nodule is pure silica. They are especially characteristic of the chalk beds, and of ancient limestones which formed on the floor of the sea, like the Helderberg Limestone of New York, Pennsylvania, Ohio, etc. When thin sections are cut through these flints, and examined under the microscope, many remnants of the shells of plants and animals are still recognizable. A nucleus is seldom found, but in some cases there is a fossil in the nodule about which the concretion doubtless formed. The spicules of sponges, shells of diatoms, and of radiolarians seem to have contributed most of the material from which flint concretions are formed. In addition to the silica there are frequently inclosed in these nodules the horny jaws of various sea worms, and a host of spiny balls the relationships of which are still unknown.
[Sandstone concretions]
There are two types of sandstone concretions, first those which are cemented with lime, and second those cemented with iron oxide. The concretions bound by lime are especially characteristic of sandstones which were laid down as river deposits, either in the channels or on the flood plains, and also the sandy deposits resulting from wind deposition. In these cases the concretions will mostly be found to have formed around some organic nucleus, most frequently about a bone, or group of bones, of some ancient animal. In this country they are mostly found in the arid and semiarid sections of the West, where the present day wind erosion exposes the harder parts of bluffs, etc.
The second type of sandstone concretion is the one in which the cement is most often limonite, less often hematite. These concretions are less dense than the lime ones, and in some cases the limonite is only precipitated at a distance from the nucleus, which has resulted in the formation of a hollow shell, filled with loose sand. This is especially characteristic of certain concretions, found in a gravel or coarse sand in the region of Middletown, Del.
[Oolites]
In large bodies of water like the sea and some larger lakes we find concretions which have formed, or are still forming, about tiny grains of sand, which are still being moved about by the waves and currents. In such cases not only are great masses of concretions formed but they have very clearly marked the concentric layering, which shows that they have increased in size, sometimes more rapidly and sometimes more slowly. Where great masses of such concretions have formed the resulting rock appears like a great mass of small eggs, whence the term oolite. The cement may be any one of several substances, but lime, silica, and hematite are perhaps the most common. Here and there are found larger or smaller masses of this oolite. In some cases it would appear that the material was precipitated by the action of bacteria. Such for instance is probably the origin of the Clinton iron ore, a bed of oolitic hematite, extending from New York State all down the Appalachian Mountains to Alabama.
[Pisolite]
[Pl. 69]
When the concretions, formed in exactly the same manner as in the case of oolite, are of a size bigger than a pea, then the rock is known as pisolite.
Other Concretions
Though less abundant concretion may form from still other substances. Hematite has been mentioned, and when concretions are made of this material, either they have been deposited by bacteria, or were formed as limonite and the water of crystallization of this latter mineral driven off.