18 BEPOET — 1881, 



arrange these plates with their flat surfaces perpendicular to the direction 

 of the pressure. Tyndall has since shown that the presence of flat flakes 

 is not necessary. He proved by experiment that pure wax could be made 

 by pressure to split into plates of great tenuity, which he attributes mainly 

 to the lateral sliding of the particles of the was over each other ; and 

 thus the result of pressure on such a mass is to develop a fissile structure 

 similar to that produced in wax on a small scale, or on a great one in 

 the slate rocks of Cumberland or Wales. 



The difficult problem of the conditions under which granite and cer- 

 tain other rocks were formed was attacked by Sorby with great skill 

 in a paper read before the Geological Society in 1858. The microscopic 

 hollows in many minei-als contain a liquid which does not entirely fill the 

 hollow, but leaves a small vacuum ; and Sorby ingeniously pointed out 

 that the rock must have solidified at least at a temperature high enough 

 to expand the liquid so as to fill the cavity. Sorby's important memoir 

 laid the foundation of microscopic petrogi-aphy, which is now not only 

 one of the most promising branches of geological research, but which has 

 been successfully applied by Sorby himself, and by Maskelyne, to the study 

 of meteorites. 



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 generally considered by 

 physicists to be untenable. Though there is still much difference of 

 opinion, the prevailing feeling on the subject has been 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 

 I'ancienne extension des Glaciers,' in which, developing the observation 

 already made by Charpentier and Venetz, that boulders had been transported 

 to great distances, and that rocks far away from, or high above, existing 

 glaciers, are pohshed 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 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 18.38, the Dinornis of New Zealand by the 

 same distinguished naturalist in 1839, the Mylodon in the same year, 

 and the Archosopteryx in 1862. 



lu America, a large number of remarkable forms have been described, 



