Rocks, let the reader be assured, have not been indiscriminately heaped together. Everything here, amidst all the apparent confusion which surrounds us, is in the most perfect order, following one uniform law of superposition. When God fixed the foundations of the earth, stretched his compass “upon the face of the deep,” and laid “the beams of his chambers in the waters,” he completed the mighty edifice agreeably to the plan which he had determined upon “from the beginning:” the different portions of the building rise one above another in regular succession; and the work, so far as we can survey the interior, displays the several _courses_ into which the materials have been thrown. These constitute what geologists call the strata of the earth, layers of varying thickness, such as our slates, sandstones, and limestones exhibit, and which nearly envelope the circumference of the globe. The order in which the strata are disposed is uniform from below upward, and this order is never inverted. From the blue slates of the Grampians to the Chalk cliffs at Dover, there is a regular succession of intermediate rocks, piled one upon another like the mason-work of our houses; and while to many there appears nothing but confusion, to the scientific eye every portion of the series, although the same ingredients enter into several classes of rocks, is as well defined and as easily recognized, as the two members at the extreme points are by the common observer. But beside the stratified rocks, there is another class of rocks equally extensive, and which occupy an important place in the economy of nature. These are the granites and whinstones of which the highest mountain ranges are usually composed. There are many subordinate varieties belonging to both classes, which are characterized by slight shades of texture and composition and distinguished by different names. One thing is common to the members of each group. They are not disposed in layers, and exhibit no lines of stratification, except in the granite rarely, throughout the entire mountain chain. These rocks occupy no fixed place in the order of superposition, but seem to be intruded in the most irregular manner among the stratified rocks, separating one bed from another, filling up fissures and rents; and binding and interlacing the various deposits more closely and firmly together. They are often composed of the fragments of other rocks, agglutinated into a compound mass by a base of clay. Remarkable changes are also produced upon all the strata where they come in contact with granite and whinstone—chalk being converted into crystalline limestone—limestone into chert—clay and sandstone into a substance as hard and compact as flint—and coal is deprived of its bitumen or the quality which renders it so useful as a combustible body.