580 THE POPULAR SCIENCE MONTHLY. 



age, a number of which undoubtedly penetrated into the shallow es- 

 tuarine lakes and salt lagoons of that period. Associated with the 

 Permian mollusca we find the Labyrinthodont Amphibian Lepidoto- 

 saurus Duffii, together with Proterosaurus Speneri and P. ILuxleyi, 

 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 so- 

 dium, 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 hav- 

 ing 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 depo- 

 sition 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 in- 

 habit 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 approxi- 

 mately fitting into each other, like sheets of paper crumpled together, 

 conveying the impression that they are somewhat tufaceous in char- 

 acter, 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. 



