In the Laurentian Highlands the metamorphosed rocks are of pre-Cambrian age; in New England and the Appalachian region they are, in part at least, of Paleozoic age; and in the Sierra Nevada and Cascade Mountains metamorphosed Mesozoic and Cenozoic rocks occur. As movements in the outer portion of the earth's crust may produce fractures in any class of rocks, and as such fractures favour the intrusion of igneous material, the metamorphic rocks may contain igneous intrusions similar to those noted above in connection with sedimentary rocks. As the stratification so marked in sedimentary beds is lacking in metamorphosed rocks, it is not to be expected that intrusions will take the form of sheets, laccoliths, etc., but rather appear as dikes with perhaps irregular branches. As the same region may experience two or more periods of metamorphism, it is evident that great complexities may arise, as, for example, when a metamorphosed terrane is penetrated by dikes and irregular intrusions and again subjected to metamorphosing conditions. These considerations lead to the suggestion that rocks metamorphosed in pre-Cambrian time, for example, would be apt to be more complex than those of Mesozoic date. In general, this has been found to be true, as is suggested by the fact that to the pre-Cambrian metamorphosed terranes, as previously stated, the name Basement Complex has been applied.
Summary.—The relation of the three great divisions into which the rocks composing North America, in common with all other portions of the known lithosphere, are divided, may perhaps be better understood when it is remembered that the igneous rocks came from below in a molten condition; that the sedimentary rocks have been formed at the surface from the débris of either igneous, metamorphic, or previously formed sedimentary beds; and that metamorphic rocks have been produced within the earth's outer crust by the alteration of either igneous or sedimentary rocks. When the heat which produced certain phases of metamorphism is sufficiently increased, greater freedom of molecular and chemical changes occur
and the material acted on passes to the condition of an igneous magma. The three great classes of rocks considered above are thus seen to be but stages in a cycle which the material of the lithosphere passes through.
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 earliest evidence of animal life on the earth, North America holds out favourable conditions.
THE CONCENTRATION OF MINERAL SUBSTANCES
The most important branch of geology treats of the substances in the earth's crust that are of direct service to man, as, for example, building stones, coal, iron, petroleum, gold, etc. Only a glance can here be given at the conditions which have led to the origin of the materials of commercial value and to their geographical distribution.
From the mode of origin of the principal classes of rocks it may be reasonably inferred that certain minerals and ores will be developed or concentrated in one class of rocks and not in the others. To a great extent the facts observed during the development of mines, etc., sustain this prediction.
In the cooling and crystallizing of igneous rocks from a state of fusion many minerals are formed, the most common being silicates of the alkaline earths, which are usually inclosed in a glassy or cryptocrystalline base. The