RHYTHMS IN DENUDATION 769 



eluded. The faet that these shallow seas were not speedily filled with 

 sediment and converted into river plains shows that erosion was dow. 

 The greater areas of the land were flat and but little above the sea. 

 Mountain axes alone were the regions of pronounced relief. The retreat 

 of the seas from the continents required a falling sealevel of not more 

 than a hundred^ or at the most a few hundred, feet. Over broad land 

 areas the elevation was so slight and drainage was so sluggish that when 

 the seas returned they commonly came to rest directly on the uneroded 

 sediments of the previous periods. During the land interval, if it was 

 more than most temporary, the layers of unconsolidated sediment above 

 baselevel were washed off and the evidences of the farther advances of the 

 previous seas were destroyed. Nevertheless this erosion was very limited 

 in depth and speaks of the limited elevation of such lands required to 

 drain the seas. During the Paleozoic and to a lesser degree in the Meso- 

 zoic the mean sealevel was evidently notably higher with respect to the 

 continents. Even in the periods of revolution, although mountain sys- 

 tems were made and the epeiric seas withdrawn, yet the continents outside 

 of the mountainous regions still stood low. This is illustrated by the very 

 broad expansion of Ordovician deposits not eroded in the Taconic dis- 

 turbance, but covered by the Silurian and later sediments. During the 

 Devonian there was intense mountain-making and igneous activity. 

 Basins were filled many thousand feet deep with continental sediments, 

 yet marine waters persisted rather widely over the interior of the Amer- 

 ican continent. Further erogenic movements in the late Mississippian 

 (Chester) and again in the late Pennsylvanian led to vast deposits of 

 waste, but these were laid down broadly as interbedded marine and conti- 

 nental deposits on the continents, not carried by rivers to the margins. 

 Even the Permian revolution was attended by so little elevation of the 

 continents that, although the seas withdrew, vast areas of sediments of 

 the previous period were preserved with their stores of coal to the age of 

 man. These coal fields are now in considerable part above sealevel, are 

 undergoing extensive dissection, and the completion of the present cycle 

 of erosion would greatly reduce the quantity of Carboniferous coal even 

 without the exploitation by man. 



On the other hand, this argument must not be pushed too far. The 

 farthest extensions of the ancient seas left thin deposits which have been 

 stripped back to later and lower baselevels by the erosion of all later time ; 

 the lesser extensions, on the contrary, have had their deposits protected 

 by blankets of later sediment. The limitation of present outcrop is con- 

 sequently no evidence of the original limitation of the formation. That 

 must be determined, in so far as it is possible, mostly by means of the 



