DEFINITIONS AND LAWS OF GEOLOGY. 7 



because the same materials in a multitude of cases have been broken up again and 

 again, and restratified, presenting for our observation only the last of the many- 

 forms through which they have passed. The oldest rocks, as well as the most recent, 

 were formed from the waste of older rocks than themselves, therefore we can never 

 see any part of the primitive earth or original solidified matter. 



§ 5. The atmospheric forces, in activity and disturbing the surface of the earth, 

 are generally combined with the aqueous, as in frost ; or the chemical, as in the 

 union with carbonic acid ; but the effects of air and wind are, by no means, incon- 

 siderable. The surface of all exposed rocks and earthy materials bear the evidences 

 of disintegration and denudation. The sun dries up the mud and cracks the earth 

 and soils, while the winds sweep the dust from roads and barren places. Grains 

 of sand, driven by the wind, will groove and polish the hardest rocks and minerals, 

 and sometimes fairly dissolve and carry away limestone and more friable substances. 

 Sand blowing is used in the arts for etching hard materials. All soils have resulted 

 from the disintegration of rocks, and when not transported, the quality depends upon 

 the character of the parent rock immediately below ; and the penetration of the soil 

 to the unaltered parent rock will reveal the different stages of the change effected by 

 atmospheric agencies, aided more or less by the effects of frost and water. The 

 winds, blowing inland from large bodies of water, carry sand from the beaches, and 

 pile it in mounds and ridges, called sand-dunes ; and the same effects are produced 

 upon the deserts, and to a greater or less extent wherever light or loose materials are 

 exposed to its action. A wind-storm blew a standing locomotive off the railroad 

 track at East St. Louis, and other storms have been known to move bodies weighing 

 several tons. The geological effects of the wind therefore are conspicuous in some 

 parts of the world, while in others they are so slight as to be quite overlooked. 



§ 6. Water is an active solvent of rocky substances, and the solvent power in- 

 creases with heat and pressure. It is also a powerful mechancial agent. It will 

 enter the minute openings in the hardest rocks, freeze, and chip up minute scales ; 

 and so it will enter larger cracks and orifices, freeze and break open large rocks, or 

 burst from ledges immense masses. Ice, freezing at the margin of lakes and ponds, 

 by expansion, crowds the loose rocks on the shores in the form of ridges of bowlders, 

 and freezing around the free rocks at the bottom in shallow water or near the shore, 

 will, when broken up by partial thawing, and assisted by the force of waves and 

 winds, transport such rocks to distant places. Mud, sand, gravel, and pieces of rock 

 are transported down stream by all rivers, and the transportation is aided by the ice 

 in the temperate and colder latitudes. On the shores of the St. Lawrence transported 

 bowlders are found weighing many tons. 



§ 7. The capacity of the atmosphere to take up aqueous vapor in suspension, 

 increases with the temperature, and when saturated the least interference with the 

 currents of the air will precipitate rain. Hence there is more rain in warmer than 

 in colder latitudes. Clouds drifting against mountains and high lands will discharge 

 rain. The rain falls upon the ground, disintegrates earthy substances, and transports 

 the disintegrated materials resulting from its own action, and from atmospheric 

 agencies, down the valleys to the ocean. It is said the Ganges annually carries to 

 the sea 6,368,000,000 cubic feet of sediment, which, being spread over the whole 

 basin of the river, comprehending 400,000 square miles, would make a layer 1-1751 

 of a foot thick. The Ganges, therefore, erodes its basin one foot in 1,751 years. 



