306 GEOLOGY. 



the reach of abundant terrigenous sediments; but within the limits 

 of the North American continent such formations were relatively 

 unimportant. 



The conditions of sedimentation during the Ordovician period were 

 somewhat different from those of the Cambrian period. Upon such 

 lands as still remained, the winds, the rains, the changes of tempera- 

 ture and the plants and animals, if then present, operated, weathering 

 the rock and preparing it for removal to the sea. But the diminished 

 area of the land within North America was attended by a decrease 

 in the amount of terrigenous sediment transported to the sea, while 

 the increased (though still shallow) depth of the epicontinental waters 

 tended to prevent its transportation far from shore. While the deposi- 

 tion of terrigenous sediment was in progress near the land, other phases 

 of sedimentation were in progress farther from it, where wind-blown 

 dust, and shells and skeletons of marine animals and plants, were 

 accumulating. 



Since the land areas of the Ordovician period were of various sizes, 

 of various sorts of rock, and presumably of various heights, conditions 

 apparently existed for the deposition of all sorts of sediments about 

 their borders. The conditions which determined variations in the 

 sediment also influenced the rate of sedimentation. The accumula- 

 tion was doubtless more rapid along the borders of such a land mass 

 as that in the eastern part of the United States, than about the smaller 

 islands at various points farther west, and sediments must have gathered 

 more rapidly on that side of any land mass towards which the larger 

 part of its drainage was directed, than on the other. 



The sediments deposited during the Ordovician period are in keep- 

 ing with these general principles. Adjacent to the broad but not 

 deep arm of the ocean which covered the larger part of the Mississippi 

 basin (Fig. 129) there appears to have been no source of abundant 

 sediments. Along the western base of the elongate tract of land (Appa- 

 lachia) in the eastern part of the United States, mud, sand, and gravel, 

 washed down from the land, were being deposited. In general the 

 coarser materials were left nearer the land, while the finer were carried 

 farther out. Alternating beds of coarse and fine sediment may indi- 

 cate either that the adjoining land was higher at some times than at 

 others, or that the climatic conditions or the vegetal covering changed, 

 or that waves and currents varied in their strength. Along this belt, 



