GEOLOGICAL IMPORTANCE OF SEDIMENTATION 457 



In conclusion, then, it may be said that during the Paleozoic the 

 eastern margin of the United States witnessed repeated uplifts and 

 occasional mountain-making movements, which poured down great, 

 quantities of sediments into a shallow and at times restricted interior 

 sea. These conditions should lead during times of rapid erosion to 

 extensive subaerial deltas ; sometimes temporary fresh- water marshes 

 or periodically inundated desert flood-plains; sometimes verdure- 

 covered swamps and plains : deltas showing great thicknesses of sub- 

 aerial beds advancing to a considerable distance into the sea, alter- 

 nating with periods of relatively more rapid subsidence or slackened 

 erosion when the sea would transgress across the subsiding delta. 

 At present the coal-beds, and the occasional presence of shells of 

 fresh-water facies in the associated dark shales, are considered the 

 only decisive evidence of such conditions, but the absence of coal- 

 beds does not prove the contrary side of the question. 



Mesozoic and cenozoic continental deposits.— The possibility of 

 the occurrence of Mesozoic continental deposits has been widely 

 recognized in both the New and Old World, especially for the closing 

 stages of the Paleozoic and the opening of the Mesozoic, when the 

 continents appear to have been broadly uplifted concomitantly with 

 erogenic movements, and conditions of coldness and aridity occurred 

 in many parts of the world, coldness and humidity in others. The 

 conditions seem to have been of a somewhat similar nature to those 

 recurring at the end of the Tertiary and enduring in a measure to 

 the present time. It does not seem necessary, therefore, in the 

 present connection to enter into a detailed discussion of the possibly 

 or probably subaerial Mesozoic deposits of Europe, Asia, and America. 

 Neither is it necessary under the present heading to discuss the 

 deposits of the Tertiary and Quaternary still enduring at the surface, 

 since it is the accumulated knowledge of these which has been used 

 as the key and the test of this discussion, and it is by a further study 

 of them that added light will be shed upon the past. 



Correction. — Through a printer's error the titles of Figs. 3 and 4 in Part 

 I, pp. 347 and 349, were transposed. 



