INTRODUCTION 11 



of water freezing in cracks. Beneath the surface, where 

 the rocks are partly or wholly filled with water, changes 

 are taking place slowly, as in a great chemical laboratory. 

 Some parts of the rock are dissolved out, leaving a spongy, 

 crumbling mass ; other parts are cemented tightly by min- 

 erals left in the pores and cracks among the grains. Still 

 deeper, where great pressure and heat are ever present, the 

 rock is mashed, welded, squeezed into sheets, and molded 

 like plastic clay. When such rock is resurrected through 

 the wearing away of the cover, it is found so changed as to 

 bear little resemblance to its original state. 



The many processes of change may be grouped under four 

 general headings. They are diastrophism, vulcanism, metamor- 

 phism, and gradation. (1) Diastrophism includes all move- 

 ments of the earth's crust of whatever sort. Some are ex- 

 tremely slow and continue for long periods, while others are 

 rapid and of brief duration. Some affect vast areas, and others 

 are local. (2) Vulcanism comprises all processes by which 

 lava and other volcanic products are forced to the surface 

 from below, and by which lava is moved from lower to higher 

 levels, even though it does not reach the surface. (3) The 

 processes by which rocks are changed, whether that change 

 results in decay or in consolidation, are included under meta- 

 morphism. (4) Gradation covers all processes which tend to 

 reduce the irregularities of the solid part of the earth. An 

 uneven surface may be made level by wearing down the high 

 places, or by building up the low ones, and so gradational pro- 

 cesses are divided into two classes. Those which seek to ac- 

 complish their end by leveling down the surface are called 

 degradational processes, in contrast to those which tend to level 

 it up, called aggradational processes. Both phases of grada- 

 tional work are done by the atmosphere, underground waters, 

 streams, glaciers, and by the waves and currents of the ocean 

 and of lakes and seas. These processes and agents are dis- 

 cussed in subsequent Chapters. 



