15. We have no intention of instituting any definite calculation with regard to the periods of time which this succession of forms of organic life may have occupied. This, indeed, the boldest geological speculators have not ventured to do. But the scientific discoveries thus made, have a bearing upon the analogies of creation, quite as important as the discoveries of astronomy. And therefore we may state briefly some of the divisions of the series of terrestrial strata which have suggested themselves to geological inquirers. At the outset of such speculations, it was conceived that the lower rocks, composed of granite, slate, and the like, had existed before the earth was peopled with living things; and that these, being broken up into inclined positions, there were deposited upon them, as the sediment of superincumbent waters, strata more horizontal, containing organic remains. The former were then called _Primitive_ or _Primary_, the latter, _Secondary_ rocks. But it was soon found that this was too sweeping and peremptory a division. Rocks which had been classed as Primary, were found to contain traces of life; and hence, an intermediate class of _Transition_ strata was spoken of. But this too was soon seen to be too narrow a scheme of arrangement, to take in the rapidly-accumulating mass of facts, organic and others, which the geological record of the earth's history disclosed. It appeared that among the fossil-bearing strata there might be discerned a long series of Formations: the term _Formation_ being used to imply a collection of successive strata, which, taking into account all the evidence, of materials, position, relations, and organic remains, appears to have been deposited during some one epoch or period; so as to form a natural group, chronologically and physiologically distinct from the others. In this way it appeared that, taking as the highest part of the Secondary series, the beds of chalk, which, marked by characteristic fossils, run through great tracts of Europe, with other beds, of sand and clay, which generally accompany these; there was, below this _Cretaceous Formation_, an _Oolitic Formation_, still more largely diffused, and still more abundant in its peculiar organic remains. Below this, we have, in England, the _New Red Sandstone Formation_, which, in other countries, is accompanied by beds abundant in fossils, as the _Muschelkalk_ of Germany. Below this again we have the _Coal Formation_, and the _Mountain Limestone_, with their peculiar fossils. Below these, we have the Old Red Sandstone or Devonian System, with its peculiar fishes and other fossils. Beneath these, occur still numerous series of distinguishable strata; which have been arranged by Sir Roderick Murchison as the members of the _Silurian_ formation; the researches by which it was established having been carried on, in the first place, in South Wales, the ancient country of the Silures. Including the lower