=Relative ages.=—The structure and relations of rocks lead to inferences as to their relative ages. In the case of stratified rocks it is obvious that overlying beds were deposited later than those below, and where there is unconformity it is evident that an interval of time elapsed between the deposition of the unconformable series. Another and in some respects more important means of telling their order of formation is found in the remains of life entrapped in the water-laid sediments. Whatever life existed in the waters in which the sediments were deposited was liable to burial, and if it was possessed of hard parts, such as bones, teeth, shells, hard integuments, etc., these parts, or at least their impressions, were likely to be preserved in the sediments. Even tracks and imprints of perishable parts are sometimes preserved. All these relics, which we call _fossils_, give indications of the kinds of life which existed when the beds were formed. The fossils of the youngest beds show that the life which existed when they were deposited was quite like that of the present time. The fossils of the next older and lower beds show greater departure from present types. This series of changes continues downward as lower and lower beds are studied, until beds at considerable depths contain no relics of existing species but, in lieu thereof, forms of more primitive types. Some of these earlier types are clearly the ancestors of more modern forms, while others seem to have no living descendants. Going still deeper, the fossils indicate life of more and more primitive types, until they depart very widely from the living forms, and seem to be but remotely ancestral. So the beds may be followed downward until the lowest, which contain distinct evidences of life, are reached. It should be understood that it is not possible to proceed directly downward through the whole succession of bedded rocks, but that the edges of the various beds may be found here and there where they have been brought to the surface by warpings or tiltings, or exposed by the wearing away of the beds which once overlay them. The full series of strata is made out only by putting together the data gathered throughout all lands, and even when this is done an absolutely complete series cannot yet be made out or, at least, has not been. =The crystalline rocks.=—The crystalline rocks which would appear if the mantle rock were removed are of two types, _igneous_ and _metamorphic_. Igneous rocks may be loosely defined as hardened lavas. Metamorphic rocks are those which are greatly changed from their original condition. Either stratified or igneous rocks may become metamorphic.