7 8 THE POPULAR SCIENCE MONTHLY. 



lishment of the Cambrian, Silurian, and Devonian systems. Our pre- 

 Cambrian 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 classi- 

 fication of the tertiary deposits, the result of the studies which he car- 

 ried on with the assistance of Deshayes and others, was published in 

 the third volume of the " Principles of Geology " in 1833. The estab- 

 lishment of Lyell's divisions of eocene, miocene, and pliocene, was the 

 starting-point of a most important series of investigations, by Prest- 

 wich 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 agen- 

 cies which is little less than the whole foundation of theoretic geolo- 

 gy 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 observa- 

 tion 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 imagi- 

 nation in recounting the wonders there revealed. As in other cases, 

 however, the realities of science have proved more varied and surpris- 

 ing than the dreams of fiction. Of the gigantic and extraordinary 

 animals thus revealed to us, by far the greatest number have been de- 

 scribed during the period now under review. For instance, the gigan- 

 tic 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 archasopteryx in 18G2. 



In America, a large number of remarkable forms have been de- 

 scribed, 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 atlanto- 

 saurus. Nor must I omit the hesperornis, described by Marsh in 1872, 

 as a carnivorous, swimming ostrich, provided with teeth, which he re- 



