10 ELEMENTS OF GEOLOGY 



broad science and that it is related closely to various sister 

 sciences. The limits of these many branches are more or less 

 artificial, and of necessity they overlap. A thoroughgoing 

 study of any one of them requires more or less knowledge 

 of some or all of the rest. Geology is indeed one great unified 

 subject, and its branches are really leading phases of the sub- 

 ject, and not distinct divisions. Little or no attention is paid 

 to them in this introductory survey of the science. In Part 

 I the materials of the earth and their arrangement, together 

 with the processes and agents which affect them, and the 

 changes which these processes and agents are bringing about 

 upon and within the earth, are discussed. This may be called 

 Physical Geology. In Part II the history of the earth is out- 

 lined briefly in the light of the principles developed in the 

 earlier chapters, and the progress of plant and animal life 

 through past ages is sketched. This is Historical Geology. 



Geologic processes and agents. Throughout the earth in- 

 cessant changes are going on, often so slowly, however, that 

 centuries are required to make their effects visible. Rocks 

 are broken, or are bent into folds, some of which appear on the 

 surface as mountain ridges. These highlands are attacked 

 in turn by wind, rain, ice, and other destructive agencies; 

 their crumbled substance is carried off by streams, winds, 

 and glaciers, only to be deposited elsewhere. Much of the 

 detritus comes to rest finally in the oceans. There, other pro- 

 cesses are at work to bind the loose grains into firm rocks, 

 which may later be elevated above the sea and even be folded 

 into more mountains. 



The processes of change are most conspicuous where air, 

 water, and rocks are in contact with one another. It is at the 

 contact of air and sea that waves are made, and these in turn 

 help to wear the land and to assort the sand and mud brought 

 down by many streams. Where air and land meet, winds blow 

 dust from one place to another, rains wash the soil, streams 

 wear their channels, and mountain crags are riven by the 

 expansion and contraction of the rocks and by the expansion 



