Igneous rocks sustain various relations to the stratified rocks, as illustrated by Fig. 2. From these relations it is possible to tell something of the order of their formation. Where the stratified rocks are broken through by lavas, it is obvious that the stratified rocks were formed first, and the lavas intruded later. Lava sheets intruded between beds of stratified rock can be told from those which flowed out on the surface and were subsequently buried, for in the former case the sedimentary rocks, both above and below the igneous rock, were affected by the heat, while in the latter case only those below were so affected. [Illustration: +Fig.+ 2.—Diagrammatic representation of the relations of igneous rock to stratified rock. The igneous rocks, represented in black, have been forced up from beneath.] More commonly than otherwise the metamorphic rocks (Fig. 3) lie beneath the sedimentary beds and are often broken through by the igneous rocks. From their position in many places their great age may be inferred, but locally, especially where dynamic action has been severe, relatively young rocks are metamorphic. [Illustration: +Fig.+ 3. The figure represents a section of the earth about 1000 miles long. The unequally thick black line at the top represents on something like its proper scale the depth of the stratified rock. The area below represents crystalline rock, largely metamorphic.] =Four great sedimentary eras.=—The water-laid series represents four great eras in the history of the earth, as shown by the relics of life imbedded in them. Beginning with the latest, these are the _Cenozoic_ (recent life), during which the life took on its modern aspect; the _Mesozoic_ (middle life), during which the life bore a mediæval aspect; the _Paleozoic_ (ancient life), during which the life belonged to older types; and the _Proterozoic_ (earlier life), during which it is inferred that much life prevailed, though its record is very imperfect. It may safely be assumed to have been more primitive than that of the Paleozoic, as it was earlier. Each of these great divisions embraced several lesser periods or epochs, and these again are subdivided more and more closely according to the degrees of refinement to which studies are carried. The chief of these subdivisions are given in the table on page 19, and others will come under consideration in the historical chapters.