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.