CHAPTER XVIII 

 THE DEVONIAN 1 PERIOD 



The passage from the Silurian to the Devonian in eastern North 

 America was without any physical break, the transition from one to 

 the other being marked by no great unconformity. In fact, so grad- 

 ual was the change that much controversy has arisen as to the exact 

 limits of the two systems. Not only was the change in the lithologi- 

 cal character and structure of the strata slight, but the life at the 

 close of the Silurian and the beginning of the Devonian was very 

 similar. Wherever the Devonian seas spread over a wider or dif- 

 ferent area from that of the Silurian the sediments were laid down un- 

 conformably on older rocks. Unconformities of this sort occur in Iowa 

 and elsewhere, but they are inconspicuous and are sometimes with 

 difficulty recognized as unconformities, since the underlying rocks 

 are horizontal and their surfaces had not been greatly roughened by 

 erosion. The lack of great relief as shown in these unconformities 

 affords an evident explanation for the fineness of the sediments in 

 the Western Interior in the closing stages of the Silurian, as well as of 

 those of the early days of the Devonian. 



Subdivisions of the Devonian. — The subdivisions of this period 

 in New York state are given below, both because it was in this state 

 that they were first studied with care in North America, and also 

 because the system is best developed there. 



Catskill and Chemung sandstones 

 Portage shale and sandstones 

 Genesee shale 

 Tully limestone 

 Hamilton shale 

 Marcellus shale 

 Onondaga limestone 

 Oriskany sandstone 

 Helderberg limestone 



' The Devonian re< eived its name from the shire of Devon, England, where a great series of 



Of llii, period 0< c ur. 



452 



