570 GEOLOGY 



here might indicate either that the deposits of the Carboniferous 

 period extended farther north than those of :iie Devonian, conceal- 

 ing the latter, or that Devonian beds, once deposited north of the 

 present border of the Carboniferous system, were worn away before 

 the deposition of the latter. Of these alternatives the latter is 

 probably correct, for near Chicago a small remnant of Devonian 

 sediment has been found in a fissure in the Niagara limestone, as 

 illustrated by Fig. 411. The limestone was apparently fissured 



Sn 



Fig. 411. Figure illustrating the occurrence of remnants of Devonian ma- 

 terial in fissures in Niagara limestone, near Elmhurst (Cook Co.), Illinois. 



before the Devonian sediments were deposited upon it. Portions 

 of the sediments fell into an open fissure, carrying with them dis- 

 tinctive fossils (fish teeth). In this protected position, the fossils 

 escaped removal. 



Igneous rocks. Igneous rocks have little representation in most 

 parts of the well-known Devonian of the continent, but in Nova 

 Scotia, New Brunswick, and Maine, and at some points in the west, 

 there are igneous rocks which appear to be of Devonian age. In 

 many places in the west, the Devonian strata have been affected 

 by dikes and intrusions of later times. 



Duration. The Devonian period was probably one of the shorter 

 periods of the Paleozoic, though its duration cannot now be stated 

 more definitely. 



Close of the Devonian 



The general period of quiet which had prevailed during the 

 period seems not to have come to an end at its close. Only in the 

 eastern part of the continent, in Maine, Nova Scotia, New Bruns- 

 wick, and the adjacent region to the north do the Devonian strata 

 appear to have been notably disturbed at the close of the period. 

 Here they are overlain unconformably by the Lower Carboniferous. 

 Elsewhere the formations of the younger system rest on those of 

 the older without stratigraphic break. There is, indeed, some rea- 

 son for associating the Upper Devonian, as here outlined, with the 

 succeeding system. 



