The superficial crust of the earth,--by which is meant only that film, compared with its diameter, which is represented by a few miles in depth--is composed of distinct mineral masses, exhibiting peculiar physical conditions and a certain order of arrangement. These rocks appear to have resulted from two dissimilar causes; in one class the action of heat is evident, and in the other we have either the slow deposition of matter suspended in water, or crystallization from solution; an aqueous origin is indicated by peculiarities of formation in all the more recent rocks. There are few branches of science which admit of speculation to the extent to which we find it carried in geology. The consequences of this are shown in the popular character of the science. A few observations are made over a limited area, and certain structural conditions are ascertained, and at once the mind, “fancy free,” penetrates the profound depths of the earth, and imagination, having “ample room and verge enough,” creates causes by which every effect is to be interpreted. Such students, generally ignorant of the first principles of physics, knowing little of mineralogy, and less of chemistry, to say nothing of palæontology, having none of the requisites for an observer, boldly assume premises which are untenable, and think they have explained a phenomenon,--given to the world a truth,--when they have merely promulgated an unsubstantiated speculation, which may have occasional marks of ingenuity, and but little else. The carefully-made observations of those who, with unwearying industry, have traversed hill and valley, marked and measured the various characters, thicknesses, inclinations, and positions of rocks; who have watched the influences of heat in changing, of water in wearing, and the results of precipitation in forming, strata; who have traced the mechanical effects of earthquake strugglings and of volcanic eruptions, and, reasoning from an immense mass of accumulated facts, deduced certain general conclusions,--are, however, of a totally different character; and it is such observers as these who induced Herschel to say truly, that “geology, in the magnitude and sublimity of the objects of which it treats, undoubtedly ranks, in the scale of the sciences, next to astronomy.”[231] The origin of this planet is involved in great obscurity, which the powers of the most gifted are unable to penetrate. It stands the work of an Almighty and Eternal mind, the beginning of which we cannot comprehend, nor can we define the period of its termination.