MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 541 



The gentleness and frequency of these movements is attested by 

 the character of the sediments and by their relations to one another, 

 as will be seen in the study of the sedimentary series. 



Reciprocal features. — These minor warpings show a notable tendency 

 to be reciprocal. If one area is bowed up, another near by is bowed 

 down. If the continents settle, the oceans rise on their borders. If 

 the land is cut down, the sea is filled up. There is an important phase 

 of this deserving especial note. Certain tracts have been slowly bowed 

 upwards into long land swells, the streams being rejuvenated and 

 degradation hastened. Adjacent tracts have been slowly bowed down- 

 wards into long parallel troughs which received the wash from the adja- 

 cent swells, and thus became tracts of exceptional sedimentation. Such 

 a tract of parallel swell and sag, if our interpretation be correct, developed 

 along the Atlantic border of North America in the Paleozoic era. By 

 the slow upward warping of the swells, the feeding-grounds of the streams 

 were maintained, and the sags were filled about as fast as they sank. 

 Thus a great depth of sediment was laid down in the course of an era 

 measured by millions of years. So in other regions, especially near the 

 borders of the continents, there have been similar reciprocal movements, 

 giving at once feeding-grounds for the streams and lodgment-grounds 

 for the sediments, side by side in parallel belts. It is a common view 

 that these belts of deep sedimentation were the forerunners of mountain 

 formation, and that they determined the formation of the mountains. 

 In view of the grounds for doubting the efficiency of so superficial an 

 agency in mountain formation, which will appear as we go on, it may 

 be well to hold this view in abeyance, and to dwell on the reciprocal 

 nature of the action, in which the upward bowing that gave the feeding- 

 grounds is as vital a factor as the sagging that accommodated the sedi- 

 mentation. It is important to recognize that in so far as the crust 

 was weak enough to yield to these gentler forces, it was not strong 

 enough to accumulate the great stresses necessary to form mountain 

 ranges, and further, that in so far as the stresses were eased by the 

 gentle warping, they could not be accumulated for the later work of 

 mountain-folding. It is nevertheless probable that the conditions 

 which located the gentle swelling and sagging also located the mountain- 

 folding. 



