8 DEFINITIONS AND LAWS OF GEOLOGY. 



The area of the Mississippi basin is 1,244,000 square miles, and the annual discharge 

 of sediment by the river is estimated at 7,471,411,200 cubic feet, an amount suffi- 

 cient to cover the whole basin 1-4640 of a foot. Therefore the Mississippi River re- 

 moves from its basin a thickness of one foot in 4,640 years. 



§ 8. The greater number of valleys in North America have been carved out 

 by the streams flowing in them at substantially the same rate of excavation that is 

 now in progress. All the valleys in Ohio, Indiana, and Illinois, have been excavated 

 by the slow process of the action of rain and the rivers. The Mississippi and all its 

 tributaries have excavated their own valleys, with the exception of a few in the 

 mountain regions. Not only have the valleys been thus excavated, but much of the 

 intervening land has been denuded of many feet of surface rocks. While the beds 

 of the older streams sink extremely slow, if at all, the valleys are gradually widening 

 by the wear and tear of rain and storm. This erosion has taken place since the close 

 of Palaeozoic time. The hills are usually terraced because the strata are of different 

 degrees of hardness and durability, the softer and more easily disintegrated are 

 gradually removed by atmospheric influences and the transporting power of rains 

 and springs, leaving the harder and more solid standing out in more or less abrupt 

 slopes and cliffs. 



§ 9. The lower limit of perpetual snow under the equator is 16,000 feet above 

 the sea, in the Swiss Alps, in latitude 46 N., it is 8,500 feet, and in the arctic and 

 antarctic regions it reaches the level of the sea. The isothermal lines, around the 

 earth, being affected by the distribution of the land and water surface and the ocean 

 currents, do not follow the degrees of latitude ; therefore, in ages past, when the land 

 and water occupied different areas, and the ocean currents moved in other routes, 

 the isothermal lines were correspondingly changed. Above the line of perpetual 

 snow there is an augmentation from year to year, and below it, during the colder 

 seasons, the snow falls many feet in thickness. An equilibrium is preserved by the 

 melting of the snow in sunshine, by occasional rains to which it is subjected, and by 

 the natural tendency to creep down the mountain side by the force of its own 

 gravity. This movement gives rise to glaciers, which follow the depressions or 

 ravines on the sides of the mountains to a considerable distance below the perpetual 

 line of snow. They move very slowly, but transport sand, gravel, and masses of 

 rock, and smooth, polish, and groove their rocky channels, because fragments of rock 

 get interposed between the glacier and the rocks of the valley. The stones carried 

 along on the ice are called the "moraines" of the glacier. 'There is always one line 

 of blocks on each side, these are called the "lateral moraines." Where there are 

 confluent glaciers the lateral moraines of the tributary glacier are carried into the 

 larger stream of ice, and are called "medial moraines." 



§ 10. The effects of glaciers upon the face of the earth are not important, not- 

 withstanding so much has been said about them, and it is evident they have not 

 been much more imposing in past geological ages than they are now. There are 

 probably no evidences of glacial action upon the continent of North America where 

 they do not now exist, except in a few places in the Rocky Mountain region, where 

 they have departed on account of the drainage of adjacent lakes, and some indica- 

 tions in the New England Mountains where they are unknown now, either because 

 that region is somewhat depressed, or because the Arctic Current does not hug the 

 shore as far south as it did in the Pliocene or Post-pliocene period. 



