cumulative height frequency curve for the Earth's surface or some part thereof. A hypsometric curve is essentially a graph that shows the proportion of land area that exists at various elevations by plotting relative area against relative height. In the hypsometric curve of the total Earth surface there exist two maxima of frequencies—at the 100-metre (109-yard) and the 4,700-metre (5,140-yard) elevations, which correlate with the mean level of the lowland continental areas and the deep-sea floor. This aspect of the Earth's surface, revealed by hypsometric analysis, supports the theory of a crust consisting of simatic (peridotitic, specific gravity about 3.3) materials under the oceans and of sialic (granitic to gabbroic, specific gravity about 2.7) materials of the continents.