Open main menu

Page:Popular Science Monthly Volume 21.djvu/850

This page has been proofread, but needs to be validated.
832
THE POPULAR SCIENCE MONTHLY.

cient seas. From among the numerous instances of this kind that present themselves to my thought, I select three—the mouths of the Rhône, the Caspian Sea, and the Dead Sea.

The map of the mouths of the Rhône shows that the lands for about twenty miles from the Mediterranean are cut up by numerous lakes, the principal of which, that of Valcares, exceeds sixty kilometres in superficial area. These lakes have been formed by the action of the alluvial contents of the river, which, being deposited on the bottom of the sea, have raised bars inclosing bodies of water. The water within the ponds, evaporating under the summer heat, is depressed in level. If the ponds were wholly isolated from each other, and had no communication with the sea, there would accumulate in each of them after a time, varying with its depth, a deposit of gypsum, and above this one of sea-salt, and so on through the series we have described; and, as the ponds are as a rule quite shallow, the saline deposits would all be thin. Things, however, do not go on thus. Most of the ponds communicate with each other and with the sea; consequently, when the water in them evaporates, the level is re-established with water that comes from the sea. In this way a pond, the bottom of which is only a few feet below the level of the sea, would become filled with saline substances if the canal of communication did not become choked. The Lake of Lavalduc, the surface of which is several yards below that of the Mediterranean, is depositing gypsum.

These facts show us saline deposits in process of formation under our very eyes, and it is not at all necessary, in order to explain their formation, to invoke changes of relief or any perturbations in the crust of the globe, but only to take an exact account of the manner in which a delta is formed, and of the circumstances that are a consequence of that mode of formation; and, although the delta of the Rhône is not one of the most extensive of the deltas of the modern period, it is one of the most remarkable and complete in respect to the spontaneous formation of saline deposits. If we should go into details, we should find a complete concordance between what is going on in the delta of the Rhône and what has been revealed in the study of the saliferous formations of past ages. Thus, saline deposits are forming in these more or less wholly isolated ponds: we have, then, a saliferous horizon in the delta of the Rhône, with the deposits generally separated; on the other hand, each of the deposits nearly always appears of a lenticular form, because the ponds of the estuaries necessarily, from the mode of their origin, assume that shape. The same principal characteristics are presented by most of the beds existing in the sedimentary deposits.

If, now, by any incident, a pond which has been closed and has deposited its gypsum is again brought into communication with the sea, life will reappear in it, and mollusks will leave their shells on top of the gypsum. If evaporation is resumed, life will disappear for a