Page:Popular Science Monthly Volume 3.djvu/595

Rh were accustomed to walk on damp surfaces of sand or mud open to the air, and the impressions left by their feet were afterward dried in the sun, before the waters flooded anew, overspread them with layers of sediment, in a manner that now annually takes place during the variations of the seasons on the broad flats of the Great Salt Lake of Utah and in other salt lakes. The occurrence of pseudomorphs of crystals of salt in the Permian beds of the Vale of Eden also helps to this conclusion, together with ripple-marks, sun-cracks, and rain-pittings, impressed on the beds. Crystals of common salt were not likely to have been deposited in an open sea, for, to form such crystals, concentration of chloride of sodium by evaporation is necessary. Deposits of gypsum, common in the Permian marls, could also only have been formed in inland waters by concentration, or on occasional surfaces of mud exposed to the sun and air, for no reasonable explanation can be offered of a process by means of which sulphate of lime can be deposited amid common mechanical sediments at the bottom of an open sea.

The question now arises how to account for the formation of the bands of magnesian limestone, sparingly intermingled with the red marls and sandstones of Lancashire and the Vale of Eden, and of that more important limestone district in the eastern half of the north of England, forming a long escarpment between Tynemouth and Nottingham. In these we have a true but restricted marine fauna, intermingled, however, with the relics of Amphibian and terrestrial life.

Let us broadly compare the marine life of the preceding epoch, that of the Carboniferous Limestone series, with the fossils of the Magnesian Limestone. The marine fauna of the Carboniferous Limestone of Britain contains about 1,500 species, most of which are mollusca (869), corals (124), echinodermata, crustacea (149), and fish (203). The Permian fauna feebly resembles that of the Carboniferous epoch, but, instead of the vast assemblage of many kinds of life found in the latter, the Magnesian Limestone of England only holds nine genera and 21 species of Brachiopoda, 16 genera and 31 species of Lamellibranchiata, 11 genera and 26 species of Gasteropoda, one Pteropod (Theca), and one Cephalopod (Nautilus). The whole comprises only 38 genera and 80 species, and all of these are dwarfed in size when compared with their Carboniferous congeners, when such there are.

I cannot easily account for this poverty of numbers and dwarfing of the forms, except on the hypothesis that the waters in which they lived were uncongenial to a true ocean fauna; and in this respect the general assemblage may be compared to the still more restricted marine faunas of the Caspian Sea and the Sea of Aral, or rather to that, a little more numerous and partly fossil, of the great Aralo-Caspian area of inland drainage, at a time when these inland brackish lakes formed part of a much larger body of water. Some of the fish of the Marl-slate have strong generic affinities with those of Carboniferous