662 



DYNAMICAL GEOLOGY. 



much like the Tertiary " Bad Lands " of the Upper Missouri. The 

 annexed cut (from Richthofen's China) represents a common scene on 

 the Hoang-ho, whose loess deposits are of great extent. The vertical 

 structure of the beds, made apparent by the erosion, may be due in 

 part to the easy percolation of waters through it ; for these waters as 

 they descend would become calcareous, from the calcareous particles 

 of the formation ; and then the deposition of the calcareous material 

 (calcium carbonate), during the intermissions in the rains, would take 

 place in vertical lines or planes. Calcareous concretions are common ; 

 and vertical tubular holes occur in much of the Chinese loess, attributed 

 to rootlets. 



The fine texture of the loess proves that it was made under comparative!}' quiet con- 

 ditions; and the height of the great terrace-plains above the river-bed of the valley, in- 

 dicates that they were formed during an era of enormous and long-continued floods as 

 well as nearly lacustrine conditions, — such floods as could have come only from the 

 melting of the ice of a Glacial era prolonged by a connection of the valley with exten- 

 sive glacial mountains. 



The valle3's of the Rhine and Danube commence in the Alps; the Mississippi received 

 waters, as the great glaciers melted, through its Rocky Mountain tributaries and the ice 

 of the mountain glaciers for a long era after that of the Lake Superior and Winnipeg 

 region had disappeared; and the Hoang-ho, of Northern China, whose modern floods 

 are vast, drains for hundreds of miles the lofty Kuenlun Mountains. A Glacial era that 

 covered the mountains of Europe with ice, would have added immensely to the ice of 

 the loftier Asiatic Mountains, since the isotherms of the globe are under general laws; 

 and it would have ended, over all the northern continents alike, in floods proportioned 

 to the amount of ice, making violent streams, and coarse beds (if there were pebbles or 

 stones within reach), in some parts, and earthy depositions wherever the waters, owing 

 to the lay of the land at the time, made lakes, or expanded into regions of quiet flow, 

 along the valleys. 



The loess of China, and even that of the Mississippi, Rhine, and other valleys, is at- 

 tributed by Richthofen chiefly to wind-drift deposition. But, as regards the Mississippi, 

 the continent on one of its sides, and for a long distance on the other, was under forests 

 or prairies, and hence afforded only locally, if at all, material for wind-transportation. 

 Moreover, the winds, besides shifting endlessly, make drift heaps and not level plains, 

 except it be in desert regions. Even a small bush will make eddies that will pile up 

 the sand. Further the lcess plains of the Mississippi are one with the plains of coarser 

 material and distinct bedding to the north, and part of the great system of terrace or 

 Champlain deposits of the continent. 



The height of flood-plains in a valley is determined approximately 

 by the height of the floods. Floods raised to different levels, would 

 tend to make plains at different levels, or terraces, in the valleys of a 

 country. If a high flood level had thus made a high flood plain or 

 terrace, other terraces might be formed at different levels below this 

 during the decline of the flood, if it were slow and intermittent in 

 progress. The enormous floods from the melting ice of a Glacial era, 

 since it would be subject to just such slowly progressing and intermit- 

 tent decline, because of the thickness of the ice, and its long continu- 

 ance about the mountains, might, therefore, leave the valleys with one 

 or several ranges of terraces. 



