THE SILURIAN PERIOD 



545 



for their exposed parts have suffered erosion, and the erosion of 

 dipping beds shifts their outcrops. In some localities there are 

 data for estimating something of the former extension of the forma- 

 tions. In Wisconsin, for example, remnants (outliers, a, b, c, Fig. 

 399) of the Niagara are found far beyond the main body of the 

 formation as it now exists. These outliers fix at least a minimum 

 limit to the original extension of the formation. 



Fig. 399. Map showing the surface distribution of Silurian, Ordovician, 

 and other formations in southern Wisconsin, northern Illinois, and east- 

 ern Iowa. -O = Cambrian; O = Ordovician; S = Niagara, and a, b, 

 c are outliers of S; M Mississippian; and C = Carboniferous. 



Igneous rocks. At few points in North America have igneous 

 rocks of Silurian age been identified. The Silurian formations are 

 sometimes affected by igneous intrusions, but the date of the intru- 

 sions is generally uncertain. Some of the igneous rocks of New 

 Brunswick are thought to be of Silurian age, and perhaps some of 

 those of Nova Scotia and Maine. 1 



Close of the period. The geographic changes at the close of the 

 Silurian were less than those at the close of the Ordovician, and 

 the Silurian system is perhaps less distinctly separated from the 

 Devonian above than from the Ordovician below. 



Climate and duration. There is nothing to indicate great 

 diversity of temperature in the Silurian period, and much to suggest 



1 Williams, H. S.. Jour, of Geol., Vol. II, pp. 16-18, and Penobscot and 

 Rockland folios, U. S. Geol. Surv. 



