lishment of the Cambrian, Silurian, and Devonian systems. Our preCambrian strata have recently been divided by Hicks into four great groups of immense thickness, and implying, therefore, a great lapse of time; but no fossils have yet been discovered in them. Lyell's classification of the tertiary deposits, the result of the studies which he carried on with the assistance of Deshayes and others, was published in the third volume of the "Principles of Geology" in 1833. The establishment of Lyell's divisions of eocene, miocene, and pliocene, was the starting-point of a most important series of investigations, by Prestwich and others, of these younger deposits, as well of the post-tertiary, quaternary, or drift-beds, which are of special interest from the light they have thrown on the early history of man.
As regards the physical character of the earth, two theories have been held: one, that of a fluid interior covered by a thin crust; the other, of a practically solid sphere. The former is now very generally admitted, both by astronomers and geologists, to be untenable. The prevailing feeling of geologists on this subject has been well expressed by Professor Le Conte, who says, "The whole theory of igneous agencies—which is little less than the whole foundation of theoretic geology—must be reconstructed on the basis of a solid earth."
In 1837 Agassiz startled the scientific world by his "Discours sur l'ancienne Extension des Glaciers," in which, developing the observation already made by Charpentier and Venetz, that bowlders had been transported to great distances, and that rocks far away from, or high above, existing glaciers, are polished and scratched by the action of ice, he boldly asserted the existence of a "glacial period," during which Switzerland and the north of Europe were subjected to great cold and buried under a vast sheet of ice.
The ancient poets described certain gifted mortals as privileged to descend into the interior of the earth, and have exercised their imagination in recounting the wonders there revealed. As in other cases, however, the realities of science have proved more varied and surprising than the dreams of fiction. Of the gigantic and extraordinary animals thus revealed to us, by far the greatest number have been described during the period now under review. For instance, the gigantic cetiosaurus was described by Owen in 1838, the dinornis of New Zealand by the same distinguished naturalist in 1839, the mylodon in the same year, and the archæopteryx in 1862.
In America, a large number of remarkable forms have been described, mainly by Marsh, Leidy, and Cope. Marsh has made known to us the titanosaurus, of the American (Colorado) Jurassic beds, which is, perhaps, the largest land animal yet known, being a hundred feet in length, and at least thirty feet in height, though it seems possible that even these vast dimensions were exceeded by those of the atlantosaurus. Nor must I omit the hesperornis, described by Marsh in 1872, as a carnivorous, swimming ostrich, provided with teeth, which he re-