Page:Popular Science Monthly Volume 9.djvu/745

Rh recent Tertiary strata. These same strata form the coast of Syrtis Major, where yet the Mediterranean extends farthest southward, and where probably was situated the last channel through which water was supplied to the sea that once covered a portion of the Sahara. That here gradual upheavals of the land have taken place, each upheaval succeeded by a protracted season of repose, is shown by the terraces, the origin of which is so well known to the geologist. Gerhard Rohlfs found these terraces as he ascended the rising ground back of Tolmita, the ancient Ptolemais. He observes that these terraces are separated from one another by levels several miles in width.

But if, turning aside from these geological considerations, we again glance at our map of relative depths, we almost everywhere find that a flat coast accompanies a shallow sea; while, on the other hand, a mountainous coast implies a sudden and precipitous inclination of the neighboring sea-bottom. This would more plainly appear if our map were on a larger scale, and had a greater number of depth-curves for the purpose of comparison. This fact might be accounted for by supposing that the comparatively sudden upheaval of the coast-hills was connected with a considerable depression of the neighboring sea-bottom, while the slow and periodic sinking of the flatter portions gave rise to submarine terraces. But, aside from this hypothesis, the representation of graduated submarine depths has a significance not to be misunderstood in geological, zoölogical, and botanical investigations.

Turning now from the sea to the land, we find in our best modern maps a number of figures indicating, as accurately as possible, the elevations; nay, even the attempt has been made, in the magnificent atlas of Switzerland, to show the elevations by means of equidistant curves. The Lehmann method of representing the surface of a country with equidistant level-lines would be, in many respects, of the highest service for the study of the earth's surface, but as yet it can be practically employed only in individual cases, partly from the want of materials, partly also on account of technical difficulties.

Cartography is a powerful aid to scientific geography, inasmuch as it arranges in true projection a great mass of heterogeneous materials, bringing it before the eye within small space, and thus making apparent relations which else could hardly be noticed. As for political geography, viz., the description of the various empires of the world, their area, provinces, population, etc., this we would regard rather as a branch of statistics than of geography proper.—Gaea.