Brick clays are low grade clays and vary greatly in composition. The main requisites are that they mould easily and bake hard at relatively low temperatures with as little warping and cracking as possible. As most clays shrink both in the air drying and in the baking, sand is added when the clay is being mixed. The color is mostly due to the presence of iron impurities. If there are iron oxides and little or no lime, the brick bakes to a red color, but if there is an excess of lime over the iron oxides, it bakes to a cream or buff color, which on vitrifying turns green.
Paving-brick clays range from surface clays, to semirefractory clays, shale being often used. The essential component is enough fluxing material, so that the bricks shall begin to vitrify, or fuse, at not too high temperatures.
Slip clays are those with a high percentage of fluxing material; so that, when baked at moderate temperatures, the surface fuses into a glassy brown or green glaze.
Adobe is an impure calcareous clay, widely used in the western United States for making sun-dried bricks.
Gumbo is a term applied to fine-grained plastic clays which shrink too much in the burning to be useful in manufactures. They can be burned to make an excellent ballast for railroads and highways. They are especially abundant in the Middle Western States.
[Loess]
This is the name given to a fine grained homogeneous clay-like material, which is a mixture of clay, fine angular fragments of sand, flakes of mica and more or less calcareous matter. It is usually without stratification, and cleaves vertically, so that, when eroded, it forms steep cliffs. Loess covers great areas in the Mississippi Valley, in the Rhine Valley, and in North Central China. By some it is thought to be an accumulation of dust in those regions where the prevailing winds were of diminished velocity and where the grass or other vegetation has served to catch and hold the material; by others it is thought of as a river and lake deposit; and by still others it is thought to be due to the combination of the two modes, wind and flood. The writer inclines to the first view expressed.
[Shale]
[Pl. 59]
When pure or impure clays, or loess, are consolidated, they are all grouped under the name shale. It usually possesses a layered or stratified structure, which makes it possible to split it into thin layers. Of all the sedimentary rocks shale is the commonest, and it may occur in all the places where clay could occur, but the most widely distributed shale is that which made the sea bottom of former times and is more or less calcareous, like the piece on [Plate 59], in which bits of shells are still visible. Shale has the same wide variation in composition as has clay, the various types being designated according to the impurity which is present, as:
argillaceous shale, made mostly of clay,