[Fr., from L. fossus, dug.] Hardened remains of animals or plants found in rocks which have been dug out of the earth. Most fossils belong to extinct species, but many of the later ones belong to species still living. The geological strata comprise three main divisions :- the Primary, the Secondary, and the Tertiary ; each of these including a number of minor divisions. In the Primary division we find corals, crustaceans, molluscs, fishes, and a few reptiles, and also an abundant flora of herbs and trees of the lower orders, found in the Carboniferous period. The Secondary age had its herbs and plants, its corals, its crustaceans, its molluscs, and its fishes; but the leading animals of this age were its huge reptiles of sea and land. It was peculiarly an age of egg-bearing animals, winged and wingless. The first birds now appeared, with teeth in their jaws, also small marsupial mammals. In the Tertiary period the mammals were wonderfully developed in size and numbers. Its mammoths and its mastodons, its rhinoceri and hippopotami,its enormous dinotherium and colossal megatherium were much larger and more numerous than the largest of existing mammals. The fossil remains of one of its elephants are still so abundant amid the frozen wastes of Siberia, that what have been not inappropriately termed " ivory quarries " have been wrought amid their bones for more than a hundred years. The western section of the United States is exceedingly rich in fossils of the Secondary and Tertiary periods, and new forms are found there annually, some of them of huge dimensions. Fos'silized Trees. In many localities collections of tree trunks converted into stone have been found. There are several of these in the United States, there being a remarkable ancient forest in Arizona, whose trees have been converted into opal or agate, with beautifully variegated colors. Some of them are six feet in diameter.

AMERICAN MASTODON, FOSSIL.

AMERICAN MASTODON, FOSSIL.