The flanks are divided into three steps: upper, middle, and lower. The division of anything into three parts is suspicious, whether it be a geologic period or a physiographic region. Such divisions usually are later replaced as more information is obtained. This is probably true of the three steps. We are more confident of the significance of the boundaries shown on the province chart than of the uniqueness of the enclosed areas, because each boundary marks a major scarp or scarp zone. The steps are defined in part in any limited area on the basis of their mean depth. In general the Upper Step ranges from 1650 to 2300, the Middle Step from 2250 to 2500, and the Lower Step from 2350 to 2800 fathoms. Just as the maximum depth of the ridge and the axis of maximum depth of the basin vary with distance along the ridge (or with latitude), so the steps vary in depth and width (Fig. 47). [Illustration: FIGURE 47.--_Axial profile of the Mid-Atlantic Ridge_ (a). Distance between province boundaries of the eastern and western flanks of the Mid-Atlantic Ridge measured along parallels of latitude. (b). Width of the Rift Valley measured at 250 fathoms and at 500 fathoms above the valley bottom and between the highest peaks of the Eastern and the Western Rift Mountains. Width in miles measured at right angles to the trend of the ridge. (c). The depth below sea level of the Rift Valley, the Rift Mountains and the western and eastern axes of maximum depth. The depths shown on this graph are in uncorrected echo-sounding fathoms.] * * * * * _Azores Plateau._--The Azores Plateau is an area of 52,000 square miles of sea floor, surrounding the Azores Islands, where the depth is less than 1000 fathoms. The Azores Islands are oriented south-southeast-north-northwest along a topographic trend which strikes off toward the Straits of Gibraltar. This topographic connection between the Azores Plateau and the southern Iberian Peninsula has been called the Azores-Gibraltar Ridge, which the present writers consider as a poorly developed mid-oceanic ridge of the same general class as the Mid-Atlantic Ridge. The Azores Plateau itself merges with the Rift Mountains of the Mid-Atlantic Ridge. The sea-floor topographic trends of the eastern part of the plateau are parallel to the known tectonic and volcanic trends of the Azores Islands (Agostinho, 1937). In the western part of the Azores Plateau trends are north-south parallel to the main trends of the Mid-Atlantic Ridge. Although both Wüst (1940a) and Tolstoy (1951) have published contour charts of the Azores Plateau, it remains largely a mystery whether the trends of the eastern Azores Plateau cross or join the axial trends of the Mid-Atlantic Ridge. The Azores Plateau or bulge is generally considered as a highly fractured tectonic uplift in which vulcanism has played a comparatively small part (Cloos, 1939). * * * * *