There is in the delta of the Indus a singular region, called the Runn of Cutch, which extends over an area of 7,000 square miles, which is neither land nor sea, but is under water during the monsoons, and in the dry season is incrusted, here and there, with salt about an inch thick, the result of evaporation. Dry land has been largely increased here, during the present century, by subsidence of the waters and upheavals by earthquakes. “That successive layers of salt may have been thrown down one upon the other on many thousand square miles, in such a region, is undeniable,” says Lyell. “The supply of brine from the ocean is as inexhaustible as the supply of heat from the sun. The only assumption required to enable us to explain the great thickness of salt in such an area, is the continuance for an indefinite period of a subsidence, the country preserving all the time a general approach to horizontally.” The observations of Mr. Darwin on the atolls of the Pacific, prove that such a continuous subsidence is probable. Hugh Miller, after ably discussing various spots of earth where, as in the Runn of Cutch, evaporation and deposit take place, adds: “If we suppose that, instead of a barrier of lava, sand-bars were raised by the surf on a flat arenaceous coast, during a slow and equable sinking of the surface, the waters of the outer gulf might occasionally topple over the bar and supply a fresh brine when the first stock had been exhausted by evaporation.”
Professor Ramsay has pointed out that both the sandstones and marls of the Triassic epoch were formed in lakes. In the latter part of this epoch, he is of opinion, that the Keuper marls of the British Isles were deposited in a large lake, or lakes, which were fresh or brackish at first, but afterwards salt and without outlets to the sea; and that the same was occasionally the case with regard to other portions of northern Europe and its adjoining seas.
By the silting up of such lakes with sediment, and the gradual evaporation of their waters under favourable conditions, such as increased heat and diminished rainfall—where the lakes might cease to have an outflow into the sea and the loss of water by evaporation would exceed the amount flowing into them—the salt or salts contained in solution would, by degrees, become concentrated and finally precipitated. In this way the great deposits of rock-salt and gypsum, common in the Keuper formation, may be accounted for.
Subsequently, by increase of rainfall or decrease of heat, and sinking of the district, the waters became comparatively less salt again; and a recurrence of such conditions lasted until the close of the Keuper period, when a partial influx of the sea took place, and the Rhætic beds of England were deposited.
The red colour of the New Red Sandstones and marls is caused by peroxide of iron, which may also have been carried into the lakes in solution, as a carbonate, and afterwards converted into peroxide by contact with air, and precipitated as a thin pellicle upon the sedimentary grains of sandy mud, of which the Triassic beds more or less consist. Professor Ramsay further considers that all the red-coloured strata of England, including the Permian, Old Red Sandstone, and even the Old Cambrian formation, were deposited in lakes or inland waters.[55]
There is little to be said of the animals which belong to the Saliferous period. They are nearly the same as those of the Muschelkalk, &c.
Fig. 85.—Pecten orbicularis.