744 PRINCIPLES OF STRATIGRAPHY 



ing marine beds are the Chemung sandstones and shales, which may 

 be regarded as the submerged continuations of the Catskill beds. 

 Oscillatory conditions are indicated by occasional intercalations of 

 the marine between the nonmarine layers. In the eastern area the 

 series is represented wholly by the nonmarine Catskill ; in the 

 western it is represented wholly by the marine Chemung. Between 

 these extremes marine Chemung is always overlain by nonmarine 

 Catskill. These relationships are shown in the diagram on page 

 742. (Fig. 156.) 



This type of overlap, though away from the source of supply, 

 is not, however, limited to nonmarine deposits. It may likewise be 

 shown by a spreading clastic shore deposit, which gradually re- 

 places a limestone of neritic origin without actual emergence of the 

 zone of shore deposits occurring. Or'a current-borne type of ter- 

 rigenous sediment may progressively replace a limestone of neritic 

 origin. Examples of such replacement are found in the changing 

 facies of the Trenton limestones, which (Grabau-4) are gradually 

 replaced westward by the spreading Utica * type of deposit, which 

 is of near-shore and probably shallow-water origin. This is illus- 

 trated in the diagram on page 743 (Fig. 157). A combination of 

 this type and the replacing by sediments of continental origin is 

 shown in Figure 158 on page 743, involving the Upper Ordovicic 

 and Lower Siluric deposits of eastern North America. 



BIBLIOGRAPHY XVIII . 



1. BARRELL, JOSEPH. 1912. Criteria for the Recognition of Ancient 



Delta Deposits. Geological Society of America Bulletin, Vol. XXIII, 



PP- 377-446. 



2. CROSBY, WILLIAM O. 1899. Archaean-Cambrian Contact near Manitou, 



Colorado. Bulletin of the Geological Society of America, Vol. X, pp. 

 141-164. 



3. GRABAU, AMADEUS W. 1906. Types of vSedimentary Overlap. Bulle- 



tin of the Geological Society of America, Vol. XVII, pp. 567-636. 



4. GRABAU, A. W. 1910. Physical and Faunal Evolution of North America 



During Ordovicic, Siluric and Early Devonic Time. In "Outlines of 

 Geological History," etc.. Chapter IV. 



5. KUMMEL, HENRY B. 1898. Annual Report of the State Geologist 



of New Jersey for 1898. New Jersey Geological Survey. 



6. PARKS, WILLIAM ARTHUR. 1899. The Nipissing-Algoma Boundary 



(Ontario). Ontario Bureau of Mines, Vol. VIII, pp. 175-196. 



* Using this term in its broader sense, in which it is practically equivalent 

 to the whole of the Trenton. 



