disintegrate the surface rocks and wash away the loose fragments and grains. This brings about a general sculpturing of the surface, forming hills and valleys, the former representing those parts of the rock formations which have not yet been worn away. The effect of this is to cause phenomena or conditions, which may at first sight appear puzzling, but are nevertheless quite simple, when the cause of them is understood. 2 Fig. 11. Horizontal beds, only the top layer showing at the surface, when the latter is flat. Let us take, for example, a section of horizontal clay beds, which originally covered an extensive area and were interstratified with sand beds. In Figs. 11 and 12, beds 1 and 3 may be taken to represent the clays. In Fig. 11, we have indicated the surface as it originally was, and in Fig. 12, the outline as it appears after the land has been exposed to weathering and erosion for an extended period. Here we see that the upper 2 Fig. 12. Horizontal beds with several layers exposed, by wearing down of the land surface. bed is left only on the highest hills, and has been removed over a large area, while No. 2 caps the smaller knolls and No. 3 outcrops in the sides of the deeper valleys. Many small areas of clay thus represent all that is left of a formerly extensive bed. If the beds had a uniform dip, the conditions may be as indicated in Fig. 13. Here, bed I appears at the summit of 2 hills, a and b, but its rise carries it, if extended, above the summit of hill c, which is capped by bed 2. If one did not know that the beds rose in that direction, it might be assumed that bed I passed into bed 2, because they are at the same level. This dipping of the layers or beds sometimes accounts for the great dissimilarity of beds at the same level in adjoining pits. Fig. 13. Inclined strata, showing rise of the bed above sea level, when followed up the slope or dip. Where a bed of clay is found outcropping at the same level on two sides of a hill it is reasonable to assume that it probably extends from one side to the other, but it is not safe to predict it with certainty, for as has been mentioned above, clay beds may thin out Sand Fig. 14. Outcrops of a clay bed on two sides of a hill and its probable extension into the same. within a short distance. Furthermore, the overlying material or overburden will become thicker towards the centre or summit of the hill, so that even if present, the clay may be economically unworkable. (Fig. 14.) Chemical Changes. Nearly all clay deposits are affected superficially, at least, by the weather. The changes are chiefly chemical, and can be grouped under the following heads: Change of color. Leaching. Softening. Consolidation. Change of Color.-Most clay outcrops which have been exposed to the weather for some time show various tints of yellow or brown. This coloration or rather discoloration is due to the oxidation or rusting of the iron oxide which the clay contains. This iron compound is usually found in the clay as an original constituent of some mineral, and rusts out as the result of weathering, so that the depth to which the weathering has penetrated the material can often be told by the color. The lower limit of this is commonly not only irregular, but the distance to which it extends from the surface depends on the character of the deposits, sandy open clays being affected to a greater depth than dense ones. The discoloration of a clay due to weathering does not always originate within the material itself, for in many instances, especially where the clay is open and porous, the water seeping into the clay may bring in the iron oxide from another layer (Pl. III, Fig. 2), and distribute it irregularly through the lower clay. The changes of color noticed in clay are not in every case to be taken as evidence of weathering, for in many instances the difference in color is due to differences in mineral composition. Many clays are colored black at one point by carbonaceous matter, whereas a short distance off the same bed may be white or light gray due to a smaller quantity of carbonaceous material. In many of the Woodbridge pits there is often a change from blue to red and white mottled and from this into red clay. This is not the result of weathering, but is due to local variations in the iron oxide contents of the different layers. It may be asked, therefore, how changes in color due to weathering can be distinguished from differences in color of a primary character. Discoloration caused by weathering begins at the surface and works its way into the clay, penetrating to a greater distance along planes of stratification or fissures, and even following plant roots as shown in Fig. 15. Where the clay deposit outcrops on the top and side of a hill, it does not follow that because the whole cliff face is discolored, the weather will have penetrated to this level from the surface, but indicates simply that the weathering is working inward from all exposed surfaces. The overburden often plays an important role in the weathering of clay, for the greater its thickness, the Wash Fig. 15. Shows how weathering penetrates a clay bed particularly along roots, cracks and joint planes. less will the clay under it be affected. This fact is one which the clay worker probably often overlooks, and, therefore, does not appreciate the important bearing which it may have on the behavior of his material. Some unweathered clays crack badly in drying or burning, but weathering seems to mellow and loosen them, as well as to increase their plasticity, so that the tendency to crack is sometimes either diminished or destroyed. If a clay which is being worked shows this tendency it will be advisable to search for some part of the deposit which is weathered, and if the clay is covered by a variable thickness of overburden, the most weathered part will be found usually under the thinnest stripping as shown in Fig. 16. Illustrations of this were found at several clay pits, but the operators, instead of recognizing the true reason for the presence of the yellow clay, presumed that it was a different bed, entirely separate from the unweathered blue. Leaching. More or less surface water seeps into all clays and in some cases drains off at lower levels. Such waters contain small quantities of carbonic acid which readily dissolves some minerals, most prominent among them carbonate of lime. In some areas, therefore, where calcareous clays occur, it is not uncommon to find that the upper layers of the deposit contain less lime carbonate than the lower ones, due to this solvent action of the percolating waters. Softening. Most weathering processes break up the clay deposits, either by disintegration or by leaching out some soluble Blue clay Yellow clay Blue clay Fig. 16. Section showing weathered (yellow) clay where the overburden is least. constituents that served as a bonding or cementing material, thus mellowing the outcrop, and many manufacturers recognize the beneficial effect which weathering has on their clay. They consequently sometimes spread it on the ground after it is mined and allow it to slake for several months or in some cases several years. The effect of this is to disintegrate thoroughly the clay, render it more plastic, and break up many injurious minerals such as pyrite (see p. 46). Although mentioned under chemical changes it will be seen that the process of softening is partly a physical one. Consolidation. This change is found to have taken place in a few deposits, notably in some of the Clay Marls, and is due to the formation of limonite crusts in the clay. At times these may |