Page:Popular Science Monthly Volume 3.djvu/596

580 age, a number of which undoubtedly penetrated into the shallow estuarine lakes and salt lagoons of that period. Associated with the Permian mollusca we find the Labyrinthodont Amphibian Lepidotosaurus Duffii, together with Proterosaurus Speneri and P. Huxleyi, both of which were true Lacertilian land reptiles.

Besides the poverty of species and the small size of the Mollusca of the true Magnesian Limestone, the chemical composition of these strata seems to afford strong hints that they were formed in an inland salt lake, the sediments of which were partly deposited through the effect of solar evaporation. Broadly stated, the rock may be said to consist of a mixture of carbonate of lime and carbonate of magnesia in proportions more or less equal, mingled with a little silicious sand mechanically deposited. The solid dolomite still contains "about one-fifth per cent, of salts soluble in water, consisting of chlorides of sodium, magnesium, potassium, and calcium, and sulphate of lime. These must have been produced at the same time as the dolomite, and caught in some of the solution then present, which is thus indicated to have been of a briny character" (Sorby). But, instead of such deposits having been formed in open sea-water, I submit that this evidence, joined to the facts previously stated, leads me to believe that our Permian dolomite was formed in an inland salt lake, in which carbonates of lime and magnesia might have been deposited simultaneously. This deposition was chiefly the result of concentration of solutions caused by evaporation, the presence of carbonate of lime in the rock being partly due to organic agency, or the life and death of the molluscs that inhabit the waters. I cannot understand how deposits of carbonate of magnesia could have taken place in an open sea, where necessarily lime and magnesia only exist in solution in very small quantities in a vast bulk of water. In the open sea, indeed, the formation of all beds of limestone is produced simply by the secretion of carbonate of lime effected by molluscs, corals, and other organic agents, and I know of no animal that uses carbonate of magnesia to make its bones.

The very lithological character of some of the strata helps to lead to the same conclusion, for, when weathered, they are seen to consist of a number of thin layers curiously bent and convoluted, and approximately fitting into each other, like sheets of paper crumpled together, conveying the impression that they are somewhat tufaceous in character, or almost stalagmitic, if it be possible to suppose such deposits being formed under water. The curious concretionary and radiating structures common in the limestone are probably also connected with the chemical deposition of the sediments.

Arguments of the same kind apply to the magnesian limestones of Lancashire and the Vale of Eden, and the miserable marine fauna in some of these beds also indicates inland unhealthy waters, while the deposits of bedded gypsum so common in the marls of the series show that the latter could not have been deposited in the sea.