The conditions which bring about these changes are still in action and are intimately associated with movements in the rocks of the earth's crust. When elevation raises a portion of the earth's crust above sea-level, erosion and redeposition ensue and sedimentary rocks are formed; the greater the elevation the more energetically the forces act which bring about denudation, transportation, and sedimentation. When depression occurs of sufficient amount to carry rocks previously at or near the surface into the zone of metamorphism, alterations follow, and in general the deeper the depression the greater the changes until metamorphism culminates in fusion, providing pressure does not counteract the influence of heat. Dynamical and chemical metamorphism may occur at less depth than purely heat metamorphism, and it may be presumed takes place in the axes of mountain ranges, even above sea-level. Such a broad view of the relations and genesis of the three great lithologic divisions of the material forming the earth's outer crust is necessary to the understanding of the conditions observed in the basal portion of the geological column, as it is termed, in which the age and order of succession of the sedimentary rocks is indicated. In certain localities, for example, the Cambrian rocks rest unconformably on a surface of metamorphic and igneous rocks--that is, the Basement Complex was raised above sea-level, eroded and subsequently depressed before the Cambrian sediments were laid upon it. In other localities the Cambrian rocks pass indefinitely into metamorphosed terranes beneath, which means that metamorphism invaded the series after the deposition of the Cambrian, and the characteristics of its junction with older rocks was obliterated. Similar relations may evidently be discovered at any horizon in the geological column. Obviously the chances of a system of stratified rocks becoming metamorphosed or of being removed by erosion, are greater the nearer their position to the base of the sedimentary series; in a similar way the chances of a sedimentary terrane becoming invaded by igneous intrusions is greater the greater its age; again, the older a sedimentary terrane the greater the chances of its becoming buried by subsequent deposition and the less the likelihood of its being exposed for study. The only position in which a sedimentary formation can maintain its integrity and be safe from destruction by erosion or transformation by metamorphism is below sea-level and above the zone of heat metamorphism; but even in this position it may have its distinctive features, including its fossils, obliterated by dynamical and chemical alterations. These suggestions are offered for the sake of indicating, as stated on a previous page, that the Cambrian and Algonkian rocks should not be considered as the first formed sediments, and that there is hope of the discovery of a rich fauna of older date than any at present known. In the search for the