682 PRINCIPLES OF STRATIGRAPPIY 



been referred to the existence of elevated ridges and cliffs in the 

 Cretacic sea in that vicinity, on which coarse Ijoulders formed the 

 only loose material, while around were accumulating finer sedi- 

 ments. (Petrascheck-65 : 26 ct scq.. Fig. /5.) 



It further appears that interruption of sedimentation and even 

 contemporaneous erosion are processes which may be active in the 

 moderate depths of the ocean, especially between depths of 200 and 

 900 meters, i. e., in the bathyal zone of the sea. Such an interrup- 

 tion may give rise to a distinct hiatus, as well as a variation in thick- 

 ness of sediments. It is not improbable that some of the breaks in 

 the geological series referred to subaerial erosions and the forma- 

 tion of disconformities may be due to the processes outlined above. 

 That such is frequently the case may perhaps be questioned, though 

 much emphasis is laid upon it by some stratigraphers and students 

 of Palaeogeography. ( IJailey Willis-106; 107.) 



The numerous breaks in the Alpine Jurassic limestone series 

 have been regarded by Neumayr and others as examples of dis- 

 continuous deposition without the occurrence of periods of dry land 

 and erosion. Currents are believed to have been the disturbing 

 agent. From the Rhretic upward irregularities in the distribution 

 of the sediment are of increasing frequency and extent ; these are 

 partly due to variations in the sea-level, partly to tectonic move- 

 ments of the suboceanic floor, and partly to the periodic want of 

 material for sedimentation (Diener). In part these breaks may 

 also be due to oceanic or tidal currents. (Andree-3.) 



Stratigraphic gaps are numerous in the geological formations of 

 North America, but it remains still to be determined which of these, 

 if any, are due to contemporaneous erosion, or to prevention of 

 deposition by currents, etc. Some of the gaps in the Ordovicic 

 limestones of the Appalachians may be explained in this manner. 

 When the great limestone series of the Palaeozoic of western 

 North America are more fully investigated, evidences of such intra- 

 formational gaps may be found. 



Persistence and Variation in Thickness of Marine Strata. 



One of the familiar phenomena confronting the field geologist 

 is the enormous variation in the thickness of strata when traced 

 from point to point. For this variation several explanations are 

 available. In the first place, it often happens that a given series of 

 great thickness is in other regions represented by only a fraction 

 of the formation, this being due to the occurrence of a constantlv 



