Moreover, the _amount_ of organic matter in the strata must have required long periods of time for its accumulation. The vegetable deposits, now converted into coal, are generally several feet thick, and often over a hundred feet, and are known to extend over several thousand square miles, both in this country and in Europe. Many of the sedimentary rocks consist almost entirely of animal remains. The mountain limestone, for instance, is eight hundred feet or more in thickness, and in some places consists of the exuviæ of encrinites and testacea. In other cases the length of time required is shown, not from the amount of organic remains, but from the evidence that they were deposited very slowly. The polishing stone called tripoli is found in beds of ten or twelve feet in thickness, and is composed entirely of the siliceous shells of animalcules, so minute that, according to the estimate of Ehrenberg, the number in a cubic inch is forty-one billions. Several other rocks, such as semi-opal and flint, are sometimes found to have a similar constitution. The time necessary for the accumulation of beds several feet thick by the shells of animalcules so minute must have been very great. Each of these facts carries us back to a period immeasurably anterior to the creation of man, as the epoch when the sedimentary deposits commenced. There are no facts in geology which point to a different conclusion. It is of the utmost importance to the geological student to familiarize himself with this principle. It will assist him in comprehending the greatness of geological changes, and in applying other principles in explanation of geological phenomena. This principle, so obvious to any one who allows himself to reason from the facts which geology presents, has sometimes been regarded as at variance with the Mosaic account of the creation. And if this account really assigns an antiquity to the earth of not more than six thousand years, the difficulty exists. The statements made by Moses are found, upon examination, to be of the most general character. They assert, in the first place, simply that "In the beginning God created the heaven and the earth." The time which elapsed after this first act, and previously to the acts of creation subsequently recorded, is not limited by the sacred narrative. It may have been during this indefinite lapse of time that God gave existence and enjoyment to a large number of animal species on the surface of the earth, and at the same time effected most of those physical changes in the crust of it which have rendered it a fit abode for intellectual and moral beings.