clay-ironstone into magnetic iron ore, coal into graphite, and lavas or volcanic ashes into hard crystalline granites, gneisses, or pyroxene rocks or hornblendic schists, according to their original composition. There may exist portions of these old rocks which have been exempt from such alteration, but hitherto we have not been able to find them, and they are probably under the ocean bed, or deeply burled beneath later rocks, while the parts exposed are precisely those which have by their crumpling and pressure, and the influence of internal heat, become most hardened and altered, and have therefore best resisted denudation. We need not therefore be astonished if any organic remains originally present in such rocks should have perished, or should have been subjected to such changes of composition and form as to have altogether lost their original characters. The searcher for fossils in such rocks has to expect that these can have been preserved only under very rare and exceptional circumstances. We have now to consider what these circumstances are, and for simplicity may suppose that we are endeavouring to discover in a crystalline limestone the remains of animals having a skeleton of limestone, as is the case with most shell-fishes and corals, and with many Protozoa and marine worms. In regard to these, we have to consider what may happen to them when they are imbedded in calcareous marl or ooze, or the limestone which results from the hardening of such materials; and we have to bear in mind that such organisms usually consist of hard, stony walls or partitions, enclosing cavities originally filled with the soft parts of the animal which may be supposed to have disappeared by decay before or during the mineralization of its skeleton.