VARIATIONS OF MARINE SEDIMENTS 683 



widening hiatus between the adjacent formations. Thus it appears 

 that the Lower ( )rdovicic series of eastern North America is repre- 

 sented by some 2.500 feet of hmestones in central Pennsylvania, 

 while only about 400 feet are found in the Mohawk Valley of New 

 York. This diminished series, however, represents only the lowest 

 part of the Pennsylvania deposits, the entire middle and upper part 

 being absent. Upon these beds rests a series of limestone 2,500 

 feet thick and representing the Middle Ordovicic, but this again is 

 represented by less than 100 feet in the Mohawk A^alley. This 

 time, however, it is the upper part of the ]\Iiddle Ordovicic which 

 is present. Thus the gap in the Mohawk \'alley cuts out all 

 but the lowest of the Lower Ordovicic and all but the highest of 

 the Middle Ordovicic. Southward the gap becomes less by the ap- 

 pearance of higher members of the subjacent and lower members 

 of the superjacent series. This is perhaps the most frequently ap- 

 plicable explanation of the differences in thickness of marine 

 formations of similar lithologic character. In the next place, there 

 is variation in thickness due to variation in character of the sedi- 

 ment. Formations of different lithic composition may show great 

 variation in thickness due to original difference in deposition. Thus 

 a sandy series is likely to be much thicker than a shale series into 

 which it passes and which was formed during the same period. 

 Again, sediments of two kinds from separate sources may over- 

 lap, and so one may thin away as the other thickens. This is espe- 

 cially the case where continental sediments overlap a marine series 

 of the same age or vice versa. The Catskill and Chemung series of 

 the LTpper Devonic of the eastern L^nited States is a case in point. 

 The Catskill continental beds are thick in the east, but die away 

 westward, while beneath the thinning cover of this series the marine 

 Chemung increases progressively from nothing in the east until it 

 alone fills the interval in western New York. But even the marine 

 series may overlap in this manner. Thus the Black shales of Port- 

 age time are thickest in Ohio, and wedge out eastward in New York 

 and Pennsylvania. Their place is taken by the sandy Portage beds 

 which have their source in the east and grow thinner toward the 

 west. 



The third cause of dift'erences in thickness is local variation 

 in the regions of sedimentation. A relatively shallow and stationary 

 area of the sea bottom may experience little sedimentation, while a 

 slowly subsiding area may receive a great supply of sediment of es- 

 sentially similar character. The Moscow shale of the Middle De- 

 vonic of western New York is a good example of this. In the 

 Genesee \'alley region its thickness is about 250 feet, while on Lake 



