850 PRINCIPLES OF STRATIGRAPHY 



ent Mississippi and Nile deltas are examples, being composed only 

 of the finest mud. The Black Devonic shale of Michigan and Ohio 

 also appears to represent a deposit of this type, as already outlined 

 in a previous chapter. 



When the continent has been worn so low that little or no sedi- 

 ment is carried into the sea, organic deposits may accumulate close 

 to the shore. Since rivers, even in low countries, are probably 

 never without their modicum of silt, it follows that pure organic 

 accumulations can be formed near shore only where large rivers do 

 not discharge. A consideration of the chalk beds of England and 

 Ireland shows them to be part of a series of coastal deposits in a 

 slowly westward transgressing sea. This is partly shown by the 

 westward overlapping of the successive members on an eroded pre- 

 Cretacic peneplain. Thus while the basal conglomerates, sands and 

 greensands of southeastern England are of Aptien age and rest dis- 

 conformably upon the Wealden, the basal Cretacic conglomerates, 

 sandstones and greensands of northeast Ireland and of Mull and 

 Morvern in Scotland are of Cenomanien age.' Here the Aptien and 

 the Gault have been overlapped, while the Cenomanien of the north- 

 west has the characteristics held by the Aptien in the southeast. 

 The Cenomanien in the southeast is a glauconitic chalk, and is suc- 

 ceeded by the pure chalk which begins with the Turonien. In the 

 northwest the Turonien is still a glauconite sand to be succeeded by 

 lower Senonien glauconitic chalk and only toward the last by pure 

 chalk. (See Fig. 146 in Chapter XVIII, page 730.) 



It is thus seen that the great mass of organic material which 

 forms the chalk was deposited in comparatively shallow water not 

 very remote from the coast, and this suggests that the land of that 

 time must have been in a state of peneplanation. The micro- 

 organisms of the chalk bear out this interpretation, for shallow 

 water benthonic forms predominate. The possibility of eolian 

 deposition of some chalk beds, mentioned in an earlier chapter, must 

 not be overlooked. 



Dissection of the Peneplain. If a peneplain is elevated, with or 

 without tilting, a new cycle of erosion commences; all the streams 

 will be revived, and they will incise their valleys, thus dissecting 

 the peneplain. At first the stream valleys are relatively insignificant 

 as compared with the broad, gently rolling upland of the elevated 

 peneplain. But as the valleys are widened, the interstream por- 

 tions are reduced and the upland dwindles into a series of ridges 

 and peaks which eventually become lowered, so that a new peneplain 

 is produced. Thus the second cycle of erosion is completed. While 

 the upland portion of the elevated peneplain is still broad, the char- 



