Breccias may result from the cementing of talus, but more often the breaking up of the material into angular fragments was due to other causes, such as crushing along a fault plane, or in the movements involved in mountain making. In such cases the breccia is of limited extent, but may occur repeatedly in the same neighborhood. Limestone, which has been crushed and then recemented, often makes a rock which takes a good polish and is used in several localities as an ornamental stone in place of marble, in fact often goes in trade circles under the name of “marble.” The breccia figured on Plate 58 is such a limestone. Soil Over most of the earth’s surface there is a covering of rock waste, the product of weathering, some of which is unassorted, and some of it sorted by water or wind. This is all termed soil. It is an ever-moving cover resulting from the decomposition of the underlying rocks, to which have been added in places layers of rock waste brought from afar by the streams. Some soils are rock waste which had been carried clear to the ocean and deposited on the floor of the sea, and is now above sea level, because the floor of the sea has been elevated. Inasmuch as the underlying rocks vary in composition, and as there are areas of transported material, it is clear that the composition of soils must vary from place to place, both as to composition and texture. Soils range from the finest, composed mostly of clay, to coarse ones, composed of sand, gravel or even boulders. Clay, the finest grained soil, is composed of particles only about ¹/₁₀₀₀th of a millimeter in diameter, of which it would take 720,000 billion particles to make a gram’s weight. Ordinary soils however have about 2 to 5 million particles to the gram. The average specific gravity of soil with the usual amount of humus in it is from 2.55 to 2.75. In this case however the specific gravity is of less importance than is the volume weight. A cubic foot of water weighs 62½ pounds, that of soil from 75 to 80 pounds, the extremes being 30 lb. for peaty soil and 110 lb. for calcareous sand. The terms “heavy” and “light,” used in agriculture do not refer to the volume weight, for clay which is actually relatively light (70-75 lb. per cubic foot) is classed as a “heavy” soil; while sand, of much greater actual weight, is classed as a “light” soil. These terms as used in agriculture refer to the ease with which the soils are worked, and to their penetrability by plant roots.