4. Let us remark at first, in addition to what has been said already (see chap. VII, n° 14), that the number of the geological periods is not arbitrarily six, since they include more than twenty-five very characteristic formations. This number marks only the great general phases. It has been adopted principally to approach to the letter of the biblical text as nearly as possible, at an epoch when it was believed to be a duty to control science by the Bible. That is why the authors of the great majority of theories of cosmogony, with a view to making their productions more easily accepted, have been forced to place themselves in accord with the sacred text. When science leans upon the experimental method, it feels stronger, and becomes emancipated. Today it is the Bible which is controlled by science.
On the other hand, exact geology, taking its point of departure only from the formation of granite rocks, does not include in the number of its periods the primitive state of the Earth. It does not occupy itself with sun, moon, and stars, nor with that portion of Genesis which belongs to astronomy. In order to deal fairly with Genesis, it agrees to add a primary period embracing this order of phenomena, which might be called the astronomical period.
Besides, the deluge period is not considered by all geologists as a distinct period, but as a transitory fact which has not notably changed the climate state of the globe, neither marked a new phase in the vegetable or animal species, since, with few exceptions, nearly the same species have been found to exist before and after the deluge. One can thus make an abstract without detracting from truth.