Led by this observation, the geologist, after comparing the rocks of different countries with one another, compared next the soils of various districts with the rocks on which they immediately rest. The _general_ result of this comparison has been, that in almost every country the soils have as close a resemblance to the rocks beneath them—as the loose earth derived from the crumbling of a rock before our eyes, bears to the rock of which it lately formed a part. The conclusion therefore is irresistible, that soils, generally speaking, have been formed by the crumbling or decay of the solid rocks,—that there was a time when these rocks were uncovered by any loose materials,—and that the accumulation of soil has been the slow result of the natural degradation (wearing away) of the solid crust of the globe. SECTION II.—CAUSE OF THE DIVERSITY OF SOILS. The cause of the diversity of soils in different districts, therefore, is no longer obscure. If the subjacent rocks in two localities differ, the soils met with there must differ also, and in an equal degree. But why, it may be asked, do we find the soil in some countries uniform, in mineral[12] character and general fertility, over hundreds or thousands of square miles, while in others it varies from field to field,—the same farm often presenting many well marked differences both in mineral character and in agricultural value? The cause of this is to be found in the mode in which the different rocks are observed to lie, one upon or by the side of the other. [12] That is, containing the same general proportions of sand, clay, lime, &c., or coloured red by similar quantities of oxide of iron. Geologists distinguish rocks into two classes, the _stratified_ and the _unstratified_. The former are found lying over each other in separate beds or _strata_, like the leaves of a book, when laid on its side, or like the layers of stones in the wall of a building; the latter form hills, mountains, or sometimes ridges of mountains, consisting of one more or less solid mass of the same material, in which no layers or strata are any where distinctly perceptible. Thus, in the following diagram, (No. 1), A and B represent _unstratified_ masses, in connection with a series of _stratified_ deposits, 1, 2, 3, lying over each other in a horizontal position. On A one kind of soil will be formed, on C another, on B a third, and on D a fourth,—the rocks being all different from each other. [Illustration: NO. 1.]