540 GEOLOGY, 



and are perhaps due in the main to the same causes, they are sufficiently 

 different in some of their dynamic aspects to be separated in treatment. 



Nearly Constant Small Movements. 



Innumerable gentle warpings have affected nearly every portion 

 of the surface of the globe at nearly all stages of its history. Not only 

 during the periods of great movements were there countless minor 

 and gentler movements, but at times of relative quiescence there were 

 slow swellings and saggings of the surface of the lithosphere. They 

 sometimes affected small areas and sometimes large ones, and they were 

 sometimes of upward phase and sometimes of downward. They were 

 the immediate agencies in locating and controlling the deposition of 

 stratified rocks, though they rest back on the great movements for their 

 working conditions. Very slow sinkings of sea-borders have permitted 

 deposition to go on in shallow water for long periods without being 

 interrupted by the local filling of the sea. Very slow swellings of land 

 tracts, relative or absolute, have permitted erosion to supply material 

 for such sedimentation for long periods without exhausting the sources. 

 Very slow upward warpings in one region and downward warpings in 

 another have shifted the borders of the land and sea, and with them 

 the areas of erosion and deposition. Thus have arisen overlaps and 

 unconformities of strata and diversities in their distribution from stage 

 to stage. Such movements may have amounted to a few inches, or a few 

 feet, or a few fathoms per century. Downward movements have some- 

 times affected a considerable section of a continent, letting in a shallow 

 epicontinental sea upon it, such, for example, as the North Sea upon the 

 northwestern border of the continent of Europe, and Hudson Bay upon 

 the northeastern part of North America. Similar movements seem to 

 have extended the seas even more widely upon the surface of the land 

 in times past, as attested by the great transgressions of the ocean-borders 

 and the great epicontinental spread of strata. Notwithstanding their 

 great breadths, the epicontinental seas were generally shallow. Similar 

 gentle warpings of upward phase rescued the bottoms of shallow seas 

 from submersion, and inaugurated erosion; or the}^ bowed base-leveled 

 lands upward, and rejuvenated their streams and inaugurated a new 

 cycle of denudation. Often they connected continents previously 

 separated by shallow straits, and thus inaugurated inter-continental 

 migrations of land life^ while they stopped inter-oceanic migration. 



