Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/322

 4. Subsequently by increase of rainfall or decrease of beat, the waters again comparatively freshened, but still remained salt, a fact proved by the occurrence of pseudomorpbous crystals of salt in sandy layers interstratified with the marls, up to, and even in, the recognized Rhaetic strata. Gypsum also occurs above the bone-bed. These circumstances may be accounted for by the close relation of the Rhaetic beds to the ordinary marly series. That Rhaetic areas got dried by temporary isolation seems certain; for sun-cracks have been observed in the strata by Mr. Bristow, and also pseudo- morphous crystals of salt.

5. During the deposition of the upper part of the Keuper marls, overlaps took place of upper across lower strata. This would necessarily take place in any deep wide lake-basin first half drained by mere evaporation, and again, by a change of conditions, gradually filled with water from which sediments were being deposited across a broader area.

For example, were the rainfall of the area drained by the Jordan to increase gradually, the basin of the Dead Sea would by degrees fill with water, and successive deposits of sediment would gradually everlap each other on the shelving slopes of the lake-basin in which solid salts had previously been deposited. There are examples of this kind of overlap in the New Red Marl of England, in Somerset, Gloucester, Hereford, and Leicester shires.

6. In the British area, sinking of the district took place at or about the time when the lake or lakes got nearly filled with sediment ; and the same may have been the case in other European areas. A partial influx of the sea took place over shallow bottoms ; and the marine life that accompanied it, and the deposits that ensued, together form the Rhaetic beds of England. These marine forms migrated from a true Rhaetic ocean, in which the Lower St.-Cassian and Hallstatt beds were deposited. If the Dead-Sea area, by increase of rainfall, got filled up, and if depression of land took place so as to admit the waters of the Red Sea or Mediterranean, analogous results would ensue ; for a marine or estuarine fauna would be superimposed in shallow water, on a set of strata containing salt in certain lower deposits. The Dead Sea, like the Keuper, is singularly destitute of remains of aquatic life.

Under these conditions, it is evident that the thin Rhaetic beds of North-western Europe might have been deposited in great part in shallow seas and in estuaries, or in lagoons, or in occasional salt lakes of small or great dimensions, separated from the sea by accidental changes in physical geography. Many years ago, while at Lyme Regis, the late Professor Edward Forbes stated to me that the fauna of the White Limestone, at that time called White Lias, reminded him, in its assemblage of forms, of the fauna of the Caspian Sea ; and this seems to be a case in point, though not in all respects strictly analogous*. The fauna of the Caspian is very small in


 * Mr. Moore, speaking of the " White Lias," considers that " the general character of the deposit is such as we might expect to find in a lagoon or inland sea