This fact is important to geology, because it enables this science to arrange all the stratified rocks in a certain invariable order,—which order indicates their relative age or antiquity,—since that which is lowest, like the lowest layer of stones in the wall of a building, must generally have been the first deposited, or must be the oldest. It also enables the geologist, on observing the kind of rock which forms the surface in any country, to predict at once, whether certain other rocks are likely to be met with in that country or not. Thus at C (diagram, No. 1), where the rock (3) comes to the surface, he knows it would be in vain, either by sinking or otherwise, to seek for the rock (1), the natural place of which is far above it; while at D he knows that by sinking he is likely to find either 2 or 3, if it be worth his while to seek for them. To the agriculturist this fact is important, among other reasons,— 1. Because it enables him to predict whether certain kinds of rock, which might be used with advantage in improving his soil, are likely to be met with within a reasonable distance or at an accessible depth. Thus if the bed D (diagram No. 2) be a limestone, the instructed farmer at E knows that it is not to be found by sinking into his own land, and, therefore, brings it from D; while, to the farmer upon C, it may be less expensive to dig down to the bed D in one of his own fields, than to cart it from a distant spot where it occurs on the surface. Or if the farmer requires clay, or marl, or sand, to ameliorate his soil, this knowledge of the constant relative position of beds enables him to say where these materials are to be got, or where they are to be looked for, and whether the advantage to be derived is likely to repay the cost of procuring them. 2. It is observed, that when the soil on the surface of each of a series of rocks, such as C, or D, or E, in the same diagram, is uniformly bad, it is almost invariably of better quality at the point where the two rocks meet. Thus C may be dry, sandy, and barren; D may be cold, unproductive clay; and E a more or less unfruitful limestone soil: yet at either extremity of the tract D, where the soil is made up of an admixture of the decayed portions of the two adjacent rocks, the land may be of average fertility—the sand of C may adapt the adjacent clay to the growth of turnips, while the lime of E may cause it to yield large returns of wheat.[13] Thus, to the tenant in looking out for a farm, or to the capitalist in seeking an eligible investment, a knowledge of the mutual relations of geology and agriculture will often prove of the greatest assistance. Yet how little is such really useful knowledge diffused among either class of men—how little are either tenants or proprietors guided by it in their choice of the localities in which they desire to live! [13] See page 94.