Belts of mobility form the most prominent features of the circum-Pacific belt. Thus they are of primary interest in any discussion of its geologic architecture. Historically, they have been the most active major features of the earth's crust. Their manner of development holds the key to the origin, growth, and real nature of continents. The circum-Pacific mobile belt is peripheral both to the high-standing, deep-rooted, approximately andesitic composition continental crust and to the broad basaltic crustal plate underlying the Pacific Ocean. It was in similar peripheral zones, now along one continental margin, then along another, that former mobile belts were initiated, developed through several hundred million years each, and were finally stabilized. Throughout geologic history, many additions were made to the margins of the continental crust by this process. Accompanying this build-up of the various continental nuclei, the always stable basaltic ocean crustal plates have, on balance, correspondingly diminished in area, and their waters have deepened. Additions to the lighter continental crust through mobile belts were effected mainly by differential subtraction of the lighter, more silicic, more alkalic constituents from the ultrabasic subcrust. This process left the latter still more basic, and added the subtracted materials to the newly forming continental crust by volcanism and other extrusives and intrusives. These materials were fed upward along several-hundred-mile-deep earth fractures or zones of disturbance, The position, depths reached, and nature of these great master "fractures" is being revealed by studies of earthquakes and volcanism, both of which are concentrated today in the circum-Pacific mobile belt. These great master "fractures" are now known to be one of the main, if not the primary controlling feature in the development life of mobile belts. Sediments derived mainly from the older, continental lands, but also from newly formed, peripheral island-arc masses accumulated in geosynclines and, in considerable part, ultimately were metamorphosed or even "granitized" before the diastrophic cycle had run its course. All stages in the diastrophic-sedimentary cycle of the typical mobile belt are represented in the circum-Pacific architecture. Thus, the various sectors of the circum-Pacific are described against the background of the sequence of stages in the life history of a typical mobile belt. After the diastrophic cycle is finished, mountains of the newly stabilized belt are eroded to their metamorphosed, intruded, and crystallized roots; thus one more section is added to the continent. Continent-building has probably been continuous by means of mobile belt activity; before one diastrophic cycle was complete others were beginning elsewhere. The new may develop in parallel fashion outside the old or may shift to another flank of the continent. In this manner continents have grown, on balance, for here and there marginal areas have also been lost to the seas. Throughout decipherable geologic history mobile belts present a similar architectural pattern. Likewise, they present a similar cross-sectional distribution, from rear to front, of rock types: sedimentary, igneous, and metamorphic. Their history and manner of growth have been strikingly similar. And so also is the distribution of petroleum in relation to them. Some now enclosed maturing sea basins of the circum-Pacific have been accumulating large quantities of sediment since Paleozoic time. These basins are of the type in which large volumes of oil have been found or will be found in the future. There seems little doubt that much oil occurs in the sedimentary sections lying below these sea bottoms; its recovery is largely an engineering problem.