APPLICATION OF PRINCIPLE OF TRANSGRESSIVE OVERLAP 593 



in part perhaps Upper Cenomanian. The Turonian, or Middle Chalk of 

 England, is already a pure white chalk, a lithic characteristic attained in 

 Ireland only in the upper Senonian. Thus a regular and progressive ad- 

 vance of the sea from southeast to northwest is indicated, with a corre- 

 sponding change in lithic character as the sea advanced. 



The Nubian sandstone of North Africa and Asia Minor appears to 

 present another case of a lithic formation rising progressively in the time 

 scale. It is the basal sandstone of the Cenomanian and later trans- 

 gression, and is probably, in part at least, a non-marine deposit reworked 

 by the advancing sea. In mount Lebanon, where this sandstone is 1,600 

 feet thick, it is succeeded by Turonian strata, while in the Lybian desert 

 Senonian chalk follows it, making the age of the sandstone itself prob- 

 ably Turonian. 



Progressive Overlap and the Black Shale Peoblem 



Wherever the relief of the land has been reduced to the condition of 

 a peneplain, the rock surface of the old land becomes mantled with the 

 products of subaerial decay. Prolonged exposure to this process results 

 in the complete disintegration of the mineral constitutents of the rocks, 

 and in the removal, by solution, of all soluble portions. When the rock 

 of the old land surface is a limestone, only the finest residual clay soil 

 will remain behind. The surface of a peneplain is preeminently char- 

 acterized by obstructed drainage conditions, and this character is the 

 more pronounced the more closely the surface of the peneplain approaches 

 that of an actual plain; hence swampy conditions may be regarded as 

 normal to the peneplain surface ; and this brings us to the conclusion that 

 the residual soils of such an area must be highly tinged with the carbon 

 of the decaying vegetation. On old limestone surfaces, the clay becoming 

 thus highly stained with carbon and the residual soil of limestone regions 

 being exceedingly fine in texftire, it follows that the resultant deposits 

 from such areas of decomposition will be a fine and uniform grained 

 black clay rock. When the sea encroaches upon such an area of residual 

 soil, the basal formation of the resulting series of deposits will be a 

 black shale, succeeded upward generally by calcareous members, since 

 the shale itself constitutes the finest clastic of shore-derived origin, and 

 any further deposits must be sea-derived — that is, organic or chemical 

 precipitates. It is by no means implied that all black mud deposits origi- 

 nate in this manner. The black muds of the protected lagoons and mud- 

 flat areas of our coasts owe their color and carbonaceous character to the 

 growth and decay of the sea grasses (Zoster a, etcetera) and the animals 

 living buried in this mud. The black shales of the Ohio Upper Devonic 



