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212 PREVAILING THEORY CONCERNING [Ch. XI. CHAPTER XI. A SKETCH OF THE PREVAILING THEORY CONCERNING THE NATURE OF THE PRIMARY ROCKS. Two classes of tlie primary rocks — the stratified and unstratified — the former of aqueous, the latter of igneous origin. — The whole Earth originally igneous — the granitic rocks the result of its secular refrigeration. — Igneous rocks of three kinds — granitic — trappean —and volcanic. — Granite a rock of all ages. — The crystalline strata associated with granite also belong to various epochs — supposed to be altered or metamorphic rocks. — The deter mination of the precise age of an individual mass of granite. — De Beau mont’s theory concerning the relative ages of mountain-chains — not generally received.—Granite protruded in a solid as well as in a fluid state. — The pre vailing theory a modification of that of Hutton — appears to be objectionable. — Lyell’s statement of this theory abridged. — The nature of the objections about to be advanced — and the order in which they are proposed to be dis cussed. Following the plan adopted in the preceding chapters, of endeavouring to make this work interesting even to those who are not professed geologists, it is now proposed to give a brief sketch of the prevailing theory concerning the nature and origin of the primary rocks. And, indeed, such a mode of proceeding may, perhaps, meet the approbation of the most experienced, as it will place the succeeding discussion on a more tangible basis, and diminish the chance of any misapprehension. According to the most generally received opinion, the pri mary rocks are divided into two grand groups or classes, the stratified and the unstratified : the former comprising various kinds of gneiss, mica-slate, and other schistose rocks; the latter, all the granitic and massive rocks commonly associated with the former. These groups are supposed to have had very different origins ; the one being of aqueous, the other of igneous formation. Thus it is presumed that the stratified