CHAPTER XVI 

 THE DEVONIAN PERIOD 



Relations to the Silurian. In North America the Silurian 

 and Devonian systems are not sharply separated from each 

 other, either by a striking unconformity or by noteworthy 

 changes in the character of the sediments. For this reason 

 there has been some dispute as to where the division should be 

 made. The fact serves to illustrate the general principle 

 that geologic time itself is unbroken and that the divisions 

 which we recognize must necessarily be somewhat arbitrary 

 and local in their application. 



At the close of the Silurian period the great central part of 

 North America seems to have been land. In many parts of 

 the country, for example, northern Illinois, Alabama, and 

 Colorado, no sediments of earlier Devonian age exist, and 

 it is thought that much of this area was land at that time. In 

 some other places, as in Iowa, an unconformity has been 

 found at the base of the Devonian system. The detection of 

 this interruption is usually difficult, inasmuch as the beds 

 below are parallel with those above; upon careful examina- 

 tion, however, the irregularity of the contact, the slightly 

 weathered surface of the uppermost Silurian beds, and the 

 abrupt change 'in the fossils serve to prove the existence of 

 the break. Such an unconformity clearly indicates two 

 things, namely, that the older rocks were not deformed, as 

 were those of New England at the close of the Ordovician 

 period, and that when the sea withdrew it left a land surface 

 of very slight relief. Had the land been high above the sea, 

 the rivers would have cut deeply into it and would either have 

 developed a very hilly surface, or, if the erosion cycle had 

 gone on to old age, the Silurian strata would have been 



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