calculate on its stability as the surest and most certain of all things.' In short, he maintained that 'there existed long periods in the history of the earth, in which there obtained conditions of things entirely different from any which obtain now--periods during which life, either animal or vegetable, could not have existed on our planet; and further, that the sedimentary rocks of this early age may have derived, even in the forming, a constitution and texture which, in present circumstances, sedimentary rocks cannot receive.' Sir Charles Lyell rejects absolutely the theory of central heat as a mode of accounting for these changes on the terrestrial surface, which are classified by geologists. He declares that no kind of rocks known to us can be proved to belong to 'a nascent state of the planet.' Disclaiming the opinion 'that there never was a beginning to the present order of things,' he nevertheless holds that geologists have found 'no decided evidence of a commencement.' Granite, gneiss, hornblende-schist, and the rest of the crystalline rocks, 'belong not to an order of things which has passed away; they are not the monuments of the primeval period, bearing inscribed upon them in obsolete characters the words and phrases of a dead language; but they teach us that part of the living language of nature, which we cannot learn by our daily intercourse with what passes on the habitable surface.' From the phenomena of precession and nutation, Mr. Hopkins, reasoning mathematically, inferred that the minimum present thickness of the crust of the earth is from 800 to 1,000 miles. This conclusion is the basis of Sir Charles Lyell's opinion respecting the Plutonic agencies which take part, or have taken part, in the formation of rocks. He shows by diagram that, if even 200 miles are allowed for the thickness of the crust, seas or oceans of lava five miles deep and 5,000 miles long might be represented by lines which, in relation to the mass of the earth, would be extremely unimportant. 'The expansion, melting, solidification, and shrinking of such subterranean seas of lava at various depths, might,' he contends, 'suffice to cause great movements or earthquakes at the surface, and even great rents in the earth's crust several thousand miles long, such as may be implied by the linearly-arranged cones of the Andes, or mountain-chains like the Alps.' To invoke the igneous fusion of the whole planet, to account for phenomena like these is, therefore, he concludes, to have recourse to a machinery 'utterly disproportionate to the effects which it is required to explain.'