“What a worthy subject for our thoughtful study is this formation of arable soil by these legions of inferior plants, obscure workers indefatigably crumbling the rock! What immense results obtained by the simplest means!” CHAPTER IV DIFFERENT KINDS OF SOIL “Four substances, mingled in very variable proportions, enter into the composition of fertile soil, or arable land, namely: sand or silica, clay, limestone, and humus, or vegetable mold. Each one of these ingredients separately would make but very poor soil, quite unsuited for agriculture; but united, mixed together, they fulfill the conditions necessary to fertility. Arable land generally contains all four, with the predominance sometimes of one, sometimes of another. The soil takes the name of its most abundant constituent. Thus have arisen the names, silicious soil, argillaceous soil, calcareous soil, and humous soil, to designate the fertile lands dominated respectively by sand, clay, limestone, and humus. Compound terms are also used. For example, when it is said of a certain soil that it is argillo-calcareous, it is meant that clay and limestone are its chief constituents. “Sand consists of particles, more or less minute, of very hard rock, sometimes opaque, sometimes as transparent as glass, and always easily recognizable by its property of emitting sparks when struck with steel. Flint and white pebbles belong to this kind of rock, which is called silex, silica, or quartz. These three expressions mean about the same. Sandy soils have little consistency, are easily permeated by water, and freely absorb the sun’s heat, which makes them very subject to drought. “The name of granite is given to a rock composed chiefly of silica and which forms whole mountains, as in central France and in Brittany. The soil formed by the gradual disintegration of this rock is sometimes called granite soil. It is not very good for agriculture. Chestnut trees prosper in it, as well as certain wild plants characteristic of this kind of land. The principal ones are the various species of heather and the purple digitalis. Heather, with its dainty little pink blossoms, carpets in richest abundance the poorest of sandy soils. The purple digitalis is a large-leaved plant whose flowers, red on the outside, striped with purple and white inside, are arranged in a long and magnificent distaff reaching almost to the height of a man. The flowers are in the shape of long tun-bellied bells or, rather, glove-fingers; hence the plant is sometimes called foxglove, sometimes lady’s fingers. “The soil composed of substances thrown up by volcanoes is also sandy, and is called volcanic soil. It is generally black and sometimes very fertile.