Although each formation may mark a very long lapse of years, each perhaps is short compared with the period requisite to change one species into another.
With marine animals of all kinds, we may safely infer a large amount of migration during climatal and other changes; and when we see a species first appearing in any formation, the probability is that it only then first immigrated into that area.
But we have seen that a thick fossiliferous formation can only be accumulated during a period of subsidence; and to keep the depth approximately the same, which is necessary in order to enable the same species to live on the same space, the supply of sediment must nearly have counterbalanced the amount of subsidence.
When we see, as is so often the case, a formation composed of beds of different mineralogical composition, we may reasonably suspect that the process of deposition has been much interrupted, as a change in the currents of the sea and a supply of sediment of a different nature will generally have been due to geographical changes requiring much time.
Nor is their rarity surprising, when we remember how large a proportion of the bones of tertiary mammals have been discovered either in caves or in lacustrine deposits; and that not a cave or true lacustrine bed is known belonging to the age of our secondary or palaeozoic formations.
But we can, I think, see why the geological formations of each region are almost invariably intermittent; that is, have not followed each other in close sequence.
Consequently formations rich in fossils and sufficiently thick and extensive to resist subsequent degradation, may have been formed over wide spaces during periods of subsidence, but only where the supply of sediment was sufficient to keep the sea shallow and to embed and preserve the remains before they had time to decay.
Jim Davis, business manager, retention, drainage, clarification at Hercules Pulp and Paper Division Wilmington, Delaware, noted that the properties of retention, drainage and formation
are related and can be influenced by many factors--including the machine set-up, furnish characteristics, and retention program.