WEATHERING. 9 



direct impact of fluids, such as air or water; but most of the fracturing and 

 abrasion effected by moving- fluids is due, not to the mechanical impact of 

 the fluid, but to the sohd masses which the fluid hurls or drags against the 

 opposing rock. 



In this complex process of leaching, decomposition, and fracturing is 

 seen the explanation of the formation of soil, subsoil, and bowlders in 

 those places where the rock of the earth's crust has been long exposed to 

 the weather. Most rocks fracture naturally into angular and rather 

 prismatic forms. The subsequent action of the weather variously modifies 

 their primitive shapes. Pieces broken off from the solid rock by natural 

 means have received many names, such as rock debris, cliff ddbris, frag- 

 mental ddbris, angular gravel, float rock, disintegrated rock, weathered 

 rock, moraine stuff, angular blocks, stones and bowlders of decomposition, 

 and, when at the foot of a cliff, talus. The words soil, subsoil, sand, and 

 clay describe certain states of weathered rock. Piece after piece is broken 

 off from the blocks into which the solid rock was originally shattered, until 

 the whole is reduced to a fine powder, known as soil ; and since the weather- 

 ing usually goes on faster at the angles, the prismatic blocks resulting 

 from the original fracture are slowly rounded at the angles and become 

 rounded bowlders of disintegration. 



Without the process of weathering there would be no soil on the earth 

 except where streams and the sea had battered the solid rock to pieces. 

 Take away the power of frost and heat to shatter and the weakening 

 effects of chemical decay, and the earth as we know it would no longer 

 exist. When first upheaved above the sea, the land might be covered by 

 sand, gravel, and clay, imperfectly fitted to be a soil. This would soon be 

 eroded away by the rains and streams, and then the continents would con- 

 sist of piles of bare rock fit perhaps to bear lichens, but with none of the 

 soils, subsoils, and drift which now bury most of the solid rock out of 

 sight and which are necessary to the existence of the higher plants and 

 animals.-' 



'The process of chemical decomposition of the rocks and soils is greatly aided hy the changes 

 of atmospheric pressure. On a grand scale these changes are due to the passage of areas of high 

 and low barometer; locally they are often due to varying pressure of the winds. As the atmospheric 

 pressure increases, air is driven down into the cavities of the earth, and when tlie pressure is 

 diminished part of this air is driven out again by expansion from within. In this manner new sup- 

 plies of oxygen and carbonic acid are continually being introduced into the rocljs and soils. The 

 process is also greatly aided by the rains. 



