A COMPARATIVE DESCRIPTION OF HYPSOGRAPHIC DATA OF SOME PLANETS The hypsographic curve and the frequency curve of heights and depths of the Earth, both externally very simple but plotted after gross generaliza tions, were first constructed by A. Penk in 1894 /7/ and W. Franbert in 1911 /5/. Subsequently, as new materials became available (the lack of data was at first particularly acute for the description of oceans), the curves were gradually made more accurate. The curves for other planets, for obvious reasons, have not yet been plotted. One of the first attempts in this direction is the plotting of the frequency curve for heights and depths on the visible side of the Moon /3/.
To disclose the deeper meaning inherent in these plots, it is advisable first to touch on some questions of the relationship between the relief and the inner structure of the epigene strata of the Earth. While small, local forms of relief are often only indirectly influenced by tectonic factors, structures of first and higher orders* generally have any one of their structural elements participating in the topography (with the exception of substructures) .
What is the reason for this feature of large topographic forms? The answer is that the inner structure develops in each case almost autonomously, and therefore differ greatly for different territories.
These differences are not only morphological and structural, but also genetic and historic-geological. While comparatively small structures differ mainly in that they are somewhat more elevated or somewhat more sunken relative to neighboring parts, large elements are characterized by sharply differing intensity of volcanic and metamorphic processes, by the degree of metamorphization, by a different composition, etc. These zones also differ qualitatively in their types of motion, which depend on "time and space coordinates".
Seas and mountains, oceans and plains ... With the exception of plains, these formations are "defective" from the viewpoint of the "orthodox" geologist, since they are devoid of the upper, granite part of the crust. For example, in marine basins, which are often prototypes of geosynclines (e. g. , the southern part of the Caspian, Black Sea, Red Sea, Gulf of Mexico, etc.), thick strata of sedimentary rocks (up to 25km on the bottom
of Caspian Sea) lie directly on a basalt substratum**. In mountains these strata often form folds whose structure sharply varies even on comparatively small stretches. At the bottom of the oceans there are virtually no sedimentary strata, and water comes almost in direct contact with basalt, and sometimes apparently peridotite, masses. There are thus great differences in the structures of different large zones. These vertical structural differences must obviously be reflected also in the relief of these zones.
We have grown so accustomed to the present-day aspect of our planet that without much conscious thought we try to fit all the enormous volume of historic-geological data available into the Procrustean bed of the contemporary terrestrial structure. On the other hand, the existence of zones which differ both in structure and in age points to diverse stages of evolution of the planet Earth.
Of considerable significance in the analysis of this evolution are such generalized characteristics of the relief as the hypsographic curve (Figure 1) and the frequency curve of heights and depths (Figure 2). The former gives a generalized, averaged profile of the planetary surface—from highest mountains to deepest troughs. Here heights and depths are laid off on the ordinate, and the corresponding areas are marked on the abscissa (from left to right). When plotting the curve, each consecutive, lower-lying area is added to the preceding value.
The frequency curve gives the distribution of heights and depths at certain intervals (for the Earth, every 200m). Since this curve is more character istic than the hypsograph, we shall refer to it first (see Figure 2). Super ficially, this curve should have only morphological meaning. However, this is not so. The frequency curve can help in answering the following question: is there any genetic relation between the principal formations of the Earth's surface—the continents and the oceans; are the oceans formed by continents, or vice versa? The only answer possible here is that these are entirely unrelated formations, and that the oceans appeared much later than the continents.