Our readers will perceive from this brief outline that, if we follow the theory of Geologists, the rocks which compose the Crust of the Earth may be conveniently divided, according to their origin, into three leading groups, the Aqueous, the Igneous, and the Metamorphic. The Aqueous are formed under water, either by the mechanical force of the water itself when in motion, or by the agency of chemical laws, or by the intervention of organic life. Hence they are naturally subdivided into three classes, the Mechanical, the Chemical, the Organic. The Igneous Rocks are produced by heat, being first melted and then allowed to cool. When this process takes place under great pressure in the depths of the Earth, the result is granite; and the granite Rocks are called Plutonic: when near the surface, through the agency of a volcano, the Rocks so formed are called Volcanic. Lastly, the Metamorphic Rocks are nothing else than Aqueous Rocks, of which the texture has been altered by the action of intense heat. As regards the relative order of position amongst these various classes of rocks, the lowest place seems uniformly to belong to the granitic or Plutonic group. It is true that the granite will often appear at the surface of the Earth; but wherever there is a series of rocks piled one above the other, the granite will always be the lowest. This assertion is based on two broad facts; first, whenever we get to the bottom of the other rocks, they are always found to rest on granite; and secondly, no other rock has ever yet been found beneath it. From this circumstance granite is conceived to be the solid foundation of the Earth’s Crust, and so is often called fundamental granite. Above the granite the Aqueous Rocks have been slowly spread out layer by layer during the long lapse of ages, now in this part of the world, now in that, according as each in its turn was exposed to the action of water. The Volcanic Rocks do not occur in any fixed order of succession. They are distributed irregularly over almost every country of the globe, occurring sometimes in the form of cone-shaped mountains, sometimes in the form of stately pillars, and sometimes in the form of massive solid walls, called Dykes, forced right through the softer Aqueous Rocks, which were deposited on the surface of the Earth before the eruption. As to the Metamorphic Rocks, which are supposed to owe their peculiar character to the contact of molten mineral matter, wherever they occur, they are found in the immediate neighborhood of some Igneous Rock.