394 



GEOLOGY. 



the upper part of the formation marked S is often Silurian (the lower 

 part being Ordovician). 



1 2 3 4 5 6 7 8 9 10 11 12 13 14 



Fig. 185. — Map of a portion of the State of New York showing the surface distribution 

 of the various Silurian and Devonian formations. 1 = Ordovician ; 2 to 5= 

 Silurian; 6 to 13 = Devonian; 14 = Mississippian. 



Sections of the Silurian in various regions are given on pp. 317-320, 

 and in the Appendix, Vol. III. 



Igneous rocks. — At few points in North America have igneous rocks 

 of Silurian age been identified. The Silurian formations are some- 



Fig. 186. — Section showing the structure of Silurian and Devonian beds in western 

 New York. 1 = Pre-Cambrian ; 2 = Cambrian (Potsdam); 3 to 6= Ordovician, 

 Calciferous, Trenton, Utica, and Hudson River respectively; 7 to 10 = Silurian, 

 Oneida and Medina, Clinton, Niagara, and Salina respectively; 11 to 14 = Devo- 

 nian, Helderberg, Hamilton, Genesee, Tortage, Catskill, and Chemung respectively; 

 and 15 = Carboniferous. 



times affected by igneous intrusions, but the date of the intrusions 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 



