'It must not be forgotten,' (these are his words) 'that the geological speculations still in vogue respecting the original fluidity of the planet, and the gradual consolidation of its external shell, belong to a period when theoretical ideas were entertained as to the relative age of the crystalline foundations of that shell wholly at variance with the present state of our knowledge. It was formerly imagined that all granite was of very high antiquity, and that rocks, such as gneiss, mica-schist, and clay-slate, were also anterior in date to the existence of organic beings on a habitable surface. It was, moreover, supposed that these primitive formations, as they are called, implied a continual thickening of the crust at the expense of the original fluid nucleus. These notions have been universally abandoned. It is now ascertained that the granites of different regions are by no means all of the same antiquity, and it is hardly possible to prove any one of them to be as old as the oldest known fossil organic remains. It is likewise now admitted, that gneiss and other crystalline strata are sedimentary deposits which have undergone metamorphic action, and they can almost all be demonstrated to be newer than the lately-discovered fossil called Eozoon Canadense.' "With all deference to one whom we acknowledge to be among the very ablest living geologists, we must say that this language strikes us as more emphatic than the state of the discussion warrants. We do not undertake absolutely to maintain the theory of central heat as explaining the formation of the granitic and metamorphic rocks, but we cannot admit, what Sir Charles seems to imply, that the time has arrived when investigation and experiment on the subject may be relinquished, and the tone of dogmatic confidence assumed. The reasonableness of permitting a certain degree of suspense of judgment regarding it becomes the more evident when we observe that Sir Charles is not prepared to maintain against astronomers that the planet was not originally fluid. 'The astronomer,' he says, 'may find good reasons for ascribing the earth's form to the original fluidity of the mass in times long antecedent to the first introduction of living beings into the planet; but the geologist must be content to regard the earliest monuments which it is his task to interpret as belonging to a period when the crust had already acquired great solidity and thickness, probably as great as it now possesses, and when volcanic rocks not essentially differing from those now produced, were formed from time to time, the intensity of volcanic heat being neither greater nor less than it is now.'