To define exactly the province of any one of the natural sciences is in no case an easy task. We define them roughly, and sufficiently perhaps for most purposes, by pointing to the classes of natural objects which the several sciences study; for example, we say that geology is the science of the crust of the earth. But such a definition is not exact or exhaustive. Geology overlaps with many other sciences; with mineralogy, when it studies mineral formations as part of the earths crust; with biology, when it studies the fossil remains of animals and plants; with astronomy or cosmogony, when it considers the conditions of the first formation of the earth's crust.