2. That good, or better land at least, than generally prevails in a district, may be expected where two formations or two different kinds of rock meet,—as when a limestone and a clay mingle their mutual ruins for the formation of a common soil. 3. That in almost every country extensive tracts of land on certain formations will be found laid down to natural grass, _in consequence of the original difficulty and expense of working_. Such are the Lias, the Oxford, the Kimmeridge, and the London clays. In raising corn, it is natural that the lands which are easiest and cheapest worked should be first subjected to the plough; it is not till implements are improved, skill increased, capital accumulated, and population presses, that the heavier lands will be rescued from perennial grass, and made to produce that greatly increased amount of food for both man and beast, which they are easily capable of yielding. The turnip soils of Great Britain are in many districts, it may be, but indifferently farmed; and the state has reason to complain of much individual neglect of known and certain methods of increasing their productiveness; but the next _great_ achievement which British agriculture has to effect, is to subdue the stubborn clays, and to convert them into what many of them are yet destined to become, the richest corn-bearing lands in the kingdom. CHAPTER VII. Soils of the Granitic and Trap Rocks.—Accumulations of transported Sands, Gravels, and Clays.—Use of Geological Maps in reference to Agriculture. —Physical characters and Chemical constitution of Soils.—Relation between the nature of the Soil and the kind of Plants that naturally grow upon it. It was stated, in the preceding lecture, (see p. 82,) that rocks are divided by geologists into the stratified and the _unstratified_.[14] The stratified rocks cover by far the largest portion of the globe, and thus form a variety of soils, of which a general description has just been given. The unstratified rocks are of two kinds—the _granites_ and the _trap_ rocks; and as a considerable portion of the area of our island is covered by them, it will be proper shortly to consider the peculiar characters of each, and the differences of the soils produced from them. [14] The unstratified are often called _crystalline_ rocks, because they frequently have a glassy appearance, or contain regular crystals of certain mineral substances; often also _igneous_ rocks, because they appear all to have been originally in a melted state, or to have been produced by fire. SECTION I.—SOILS OF THE GRANITES AND TRAP ROCKS.