During the formation of these strata there appears to have been both birds and quadrupeds of many kinds, together with a reptile much resembling a frog, but of great size, being five or six feet long, called the "Labyrinthodon" (fig. 20). The oxide of iron, or some other agent appears to have prevented the fossilisation of the inhabitants of these strata, for but few of their organic remains have been preserved; but very curious evidences of their existence nevertheless remain, in the impressions of their feet upon the ground they trod (fig. 21), which appears to have been a moist clay or mud peculiarly adapted to receive impressions, and which having been in many places covered over with a stratum of fine sand, and then abandoned by the sea, the whole have hardened into stone, and being now separated, the one contains their footprints and the other perfect casts of them! Nor are these foot-marks all that these sandstones have to tell us of their day; for the ripples of the waves, and even the little pits made by drops of rain as they fell, are in this most marvellous manner preserved, forming objects of wonder and admiration for us mortals to contemplate, and themes whereon the devout mind may pour out its tribute of praise to their Great Author. How evident it is that the Creator designed beforehand that we should search for these hidden evidences of His handiwork, or for what purpose were they thus stored up and preserved? "Seek, and ye shall find, knock, and it shall be opened unto you," are the words of God, and they apply as fully to the material wonders of His works as to the mysteries of His revealed Word. As the strata below the new sandstone formation was called the "Carboniferous" system, from its containing much carbon in the form both of coal and carbonic acid, so this has been called the "Saliferous" system, from the occurrence in many places of strata of "rock-salt" or crystallised chloride of sodium, and (where the rain finds its way down and dissolves it) of brine springs; these (in England) exist chiefly in Cheshire and Warwickshire, but in Poland and Hungary they exist on a much larger scale, the rock-salt being nearly a thousand feet thick. It has been said that these strata of salt were formed by the evaporation of salt lakes, but it is much more probable that salt is one of the natural materials of the earth, and that both salt lakes and oceans have become salt from dissolving out these strata wherever they have come into contact. The next sediment deposited over the new red sandstone is called the "Lias," a sort of limestone mixed with clay of a blueish-grey colour, and upon this has is again deposited the various strata known as the "Oolite" (Roe-stone) system, from its appearance resembling the roe of a fish, it consists of small rounded particles of limestone set in a cement of the same substance worn down fine. [Illustration: FIG. 22.--AMMONITE (Henlyi).] [Illustration: FIG. 23.--PENTACRINITE.]