2i8 MOVEMENTS AND DEFORMATIONS 



their courses, and the movements of continents are yet slower; but 

 far apart as these contrasted movements are, they are doubtless 

 associated in cause. Many earthquake shocks are but incidents in 

 the formation of mountains or in the movements of continents. 



Great movements may be classified variously, as (i) continent- 

 making, (2) plateau-forming and (3) mountain-folding; as (i) gen- 

 eral (epeirogenic) and (2) concentrated (orogenic}; as (i) vertical 

 and (2) horizontal; and dynamically, as (i) thrust and (2) stretching 

 movements. These distinctions are analytical conveniences, but 

 the various types of movement are not exclusive of one another, 

 for continental movements may involve mountain-making, verti- 

 cal movements involve horizontal movements in most cases, and 

 stretching usually attends the outward bends of thrust folds. 



Present movements. Observations on seacoasts show that 

 some shores are rising slowly and some sinking slowly, relative to 

 sea level. It is not certain what these movements are, relative to 

 the center of the earth. Theoretically all parts of the coast may be 

 sinking, some faster than others, while the ocean-surface goes down 

 at an intermediate rate; or all parts may be rising, but at different 

 rates; or again some lands may be actually rising relative to the 

 center of the earth, and others sinking, while the ocean-level has 

 an intermediate movement or none at all. We are accustomed 

 to take the sea-level as a standard, as though it were stationary, 

 which is probably not the fact. A general shrinkage of the earth 

 is probably going on, carrying down the surface of both land and sea. 

 It is possible that the shrinkage is so great that many of the upward 

 warpings and foldings do not equal it. If this is true, most move- 

 ments are really toward the earth's center. There is a popular 

 predilection for regarding earth movements generally as "up- 

 heavals," and for regarding the rigid land as moving and the 

 mobile sea-level as fixed. In reality, the sea is an extremely adap- 

 tive body which settles freely into the depressions of the lithosphere, 

 and is shifted with every warping of the latter. Whatever change 

 affects the capacity of the sea basins affects the sea-level. If the 

 basins are increased, the sea settles deeper into them; if they are 

 decreased, the sea spreads out more widely over their borders. The 

 one thing that gives a measure of stability to the sea-level is the 

 fact that all the great basins are connected, and so an average is 

 maintained. For this reason the sea-level is the most convenient 

 basis of reference, and has become the accepted datum-plane, not- 



