The only remaining question is "What was going on in the earth at the same time?" Our materials for answering this question are but scanty. So great an alteration in the sources of light and heat must have involved great physical changes on the earth's surface, and there is reason to believe that great mechanical forces were at work producing vast changes in the relations of land and water. "It has long been the opinion of the most eminent geologists that the coalfields of Lancashire and Yorkshire were once united, the upper coal measures and the overlying Millstone Grit and Toredale Bocks having been subsequently removed by denudation; but what is remarkable is the ancient date now assigned to this denudation, for it seems that a thickness of no less than 10,000 feet of the coal measures had been carried away before the deposition of even the lower Permian Rocks, which were thrown down upon the already disturbed truncated edges of the coal strata." [Footnote: Lyell, Geology for Students, p. 377.] And this is but a single instance. During the interval between the deposition of the coal measures, which seem to belong to the third, and the Saurian remains which mark the fifth day, we have the Permian and Triassic Rocks, of which the Magnesian. Limestone and the new Red Sandstone are the most important representatives in England. Till a very recent period it was thought that these rocks belonged to a period remarkably destitute of animal life, very few fossils having been found in them. Recently, however, some very rich deposits have been found in the Tyrol, belonging to this period, but they are only local. Of the Permian formation Sir C. Lyell says, "Not one of the species (of fossils) is common to rocks newer than the Palaeozoic." [Footnote: Geology for Students, p. 369.] This was not then a time for the origination of new forms of life. In the Trias, however, the new development of life, which was to attain its full dimensions on the fifth day, begins to open upon us. The earliest Saurian fossils are found, and the rocks still present us with impressions of the feet of reptiles and birds, which walked over the soft seashore, and left footprints, which were first dried and hardened by the sun and wind, and then filled up with fresh sand by the returning tide, but never entirely coalesced with the new material. At the close of this period the first traces of mammalian life occur, in the shape of teeth, which are supposed to have belonged to some small Marsupial quadrupeds, and in America the whole lower jaws of three such animals have been discovered; but no other remains have as yet been traced.