570 CENOZOIC TIME. 



so that marine shell deposits were there formed, — an event which 

 had not happened since the Lower Helderberg period in the Silu- 

 rian age (p. 228). 



The course of the movements was, therefore, diverse from that 

 of earlier time, and their results were also widely different. 



A cause of this transfer of the area of oscillation to the high latitudes may be 

 this : that the accumulation of the successive formations over the temperate and 

 tropical zones, and the elevation of the lofty mountains of the globe across the 

 same regions, together with the metamorphism of part of the rocks, had so 

 weighted, ribbed, and stiffened the crust in these parts that it was less yielding 

 to any oscillating force than that of the regions more to the north, which till 

 now had been the comparatively stable area. The series of rocks has less thick- 

 ness and completeness in the higher latitudes than in the middle and lower, and 

 the mountains less height. 



During the Post-tertiary some of the most prominent dynami- 

 cal agencies on the globe were intensified vastly beyond their former 

 power : — 



(1.) Owing to the completion of the great mountain-chains and 

 the expansion of the continents, the heights for condensing moist- 

 ure and the extent of slope for its accumulation into rivers had 

 augmented many fold. Moreover, through the union of lands be- 

 fore separated by seas into one continental area, the rivers draining 

 immense regions were for the first time united into a common 

 trunk. The Post-tertiary was therefore eminently the era of the first 

 grand display of completed river- systems, — of the first Amazon, Missis- 

 sippi, Ganges, Indus, Nile, etc. 



(2.) The elevation of the mountains to snowy altitudes introduced 

 rivers of ice, or glaciers, among dynamical agencies, or gave them 

 vastly increased extension. 



(3.) The increase of cold, and the existence finally of true frigid 

 zones, due partly, at least, to an increase of polar lands after the 

 close of the Cretaceous period and through the Tertiary, added to 

 the extent of glaciers, rendering them possible in regions where 

 otherwise they could not have existed. 



(4.) The cause last mentioned also gave origin to icebergs. 



Great rivers, glaciers, and icebergs were especially characteristic 

 forces of the Post-tertiary ; and the ice accomplished what was im- 

 possible for the ocean. In no other period of geological history 

 have so large masses of stone been moved over the earth's surface 

 as in the Glacial and later epochs. 



These Post-tertiary agencies were active everywhere over the 

 continents, putting the finishing-strokes to the nearly completed 

 globe. There was a development of beauty as well as utility in all 



