In North America, as is indicated roughly on the map forming Plate IV, metamorphic rocks occur at the surface over a great region in eastern and northeastern Canada, in Labrador and Newfoundland, in the New England States, and thence southward along the eastern side of the Appalachians. Other extensive regions occupied by similar rocks occur in many of the ranges of the Pacific mountains, from Alaska to Panama, and are known in the West Indies. Not only do the metamorphosed rocks outcrop at the surface over large areas, but, as may be inferred from such outcrops, as well as from the records of numerous borings, underlies nearly the entire extent of the sedimentary formations. The basal portion of the continent, with the exception of certain areas where igneous rocks occur, is formed of metamorphosed terranes. So generally is this true, that it is safe to say that if a boring is begun at any locality on the continent where sedimentary beds occur, and is continued downward until the sedimentary rocks are passed through, metamorphic terranes will be found beneath. The same is true also where the surface is composed of lava-sheets. The exceptions, where metamorphosed rocks do not occur beneath sedimentary or volcanic beds, are when igneous intrusions or ancient lava-flows are present at a depth. In the brief description given of the Archean system on a preceding page, it was stated that the rocks composing it are largely metamorphic. But rocks of practically any age may be altered in the several ways mentioned above, and the resulting gneisses, schists, etc., be indistinguishable from those of the Archean. In fact, some of the metamorphosed rocks of North America, as certain gneisses, schists, etc., of the Sierra Nevada and Cascade Mountains, are known to be of Mesozoic and even Cenozoic age. In speaking of the growth of North America, and again in connection with the distribution of volcanic mountains, it was shown that there has been a progressive migration of the field of action of the forces which upheave the rocks so as to form land areas, and also of the movements in the rocks which produce fractures and lead to the origin of volcanoes. In a similar way the sphere of influence of metamorphism as indicated by the age of the transformed rocks in various regions has in a general way migrated from east to west across the continent.