Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/682

 Crassatella complicata, and Trigonia Goldfussi make their appearance. These rocks, it will be remembered, I have looked upon as the uppermost of the Zwartkops- and Sundays-River strata. At the Government Saltpan, Trigonioe and Turritelloe are mixed with the remains of Cidaris — while in the section near the Salt Vlei (Sect. S, fig. 6, 3) we find Crassatella complicata and T. Goldfussi, together with a Gervillia (shells found in the uppermost Sundays-River strata), imbedded in a sandstone, immediately above which are found stratified clays containing shells which, together with Crassatella complicata, are associated with Cidaris pustulifera, the characteristic shell of the fossiliferous zones of the Saliferous Strata, and, apparently, the Trigonioe, Ammonites, and their associates are absent. This explanation proves, I think, the regular sequence of the shells.

Mr. Tate, in his paper " On some Secondary Fossils from South Africa," says*: — "Species of this type (Cidaris pustulifera) exist at the present time, and are found in the Tertiary and Cretaceous rocks ; species of the type with crenulated bosses characterize Oolitic deposits. There are, however, some exceptions to these rules, and for the present the African species may be regarded as another exceptional example." If I understand this rightly, some species of Cidaris (this fossil, remember, is associated with Crassatella complicata) are indicative of Cretaceous rocks ; but this is an " exceptional case." If, then, these same deposits are placed in the position which I believe is their true one, the exceptional condition vanishes ; they take their right place, and prove that the law which regulated, with regard to periods, the development of particular races was the same in the southern as in the northern hemisphere.

Origin of the Salt. — The author thinks that the Trigonia-series, having been deposited in open sea, would be less likely to contain salt than shallow-water beds succeeding them and formed in narrowing creeks and lagoons. The saliferous series he believes to have thus succeeded the Trigonia-beds, and to have become impregnated with salt. At the present time the same causes are at work on a smaller scale, in the mouths of some of the minor rivers of South Africa, where the entrances are blocked up with sand and thus communication with the ocean is cut off, except when broken through occasionally by a freshet after heavy rains. During the time that these mouths are thus land-locked the evaporation is more rapid than the supply ; and as a natural consequence, the water in the enclosed basins becomes more intensely salt than that of the neighbouring sea. The deposits formed within them are saturated with this extra-saline fluid, so that when a flood bursts through the opposing barrier, and the water of the imprisoned river falls with the tide, banks of brackish mud are exposed in many places ; and these soon prove themselves to be saliferous deposits ; for, as they dry in the sun, they become covered with a white saline efflorescence. Such, one cannot help believing, is the explanation of the mode of formation, although on a grander scale, of the saliferous strata we have been considering.


 * Quart. Journ. Geol. Soc. vol. xxiii. p. 163.