LAURENTIAN SYSTEM. 17 



Brought forward, 102,050 feet. 



. f Oriskany Group, 300 " 



(3 Upper Helderberg Group, 900 " 



'2 I Hamilton Group, 1,400 " 



g 1 Portage Group, 1,400 " 



Chemung Group, 3,000 " 



1 L Catskill Group 7,500 " 



, f Waverly Group, 500 



^ I Burlington Group, 500 



8 g I Keokuk Group, 200 



3 I Warsaw Group, 100 



g S St. Louis Group, 400 



w [Kaskaskia Group, 720 



In Pennsylvania, where the Subcarboniferous can not be 

 separated into Groups, there is a thickness of 5,000 feet, and 

 in Nova Scotia 6,000 feet. 



JL w f Carboniferous Conglomerate, or Millstone Grit, 6,000 



S '5 \ Coal Measures, 10,000 



.o u (.Permian Group, 1,000 



Triassic not divided into Groups, 25,000 



Jurassic not divided into Groups, , 10,000 



Dakota Group, 1,000 



Fort Benton Group, 900 



Niobrara Group, 500 



Fort Pierre Group 1,200 



Fox Hills Group, 4,000 



Fort Union or Laramie Group, 7,000 



. f Eocene, 15,000 



1 b I Miocene, 3,000 



.2 j Pliocene, 3,000 



*" L Post-pliocene, 1,000 



Total, 211,150 feet. 



CHAPTER II. 



SYSTEM. 



30. THE Laurentian System was so named from the Laurentian Mountains, 

 and not from the St. Lawrence River. The name was applied to the metamorphic 

 rocks of Canada as a scientific term, by Sir William Logan, in the Report of Progress 

 of the Geological Survey of Canada for the years 1852-53. His special study 

 of these rocks began as early as 1846. He applied the name to all rocks lower than 

 the Potsdam; but Emmons had preceded him in defining the Taconic System, which 

 rests uncomformably upon the rocks that comprise nearly all which Logan studied ; 

 and hence the Laurentian is confined to the rocks below the Taconic. The rocks 

 consist of sedimentary strata altered to a highly crystalline condition great vertical 

 thicknesses of gneiss and granitoid rock's, separated by masses of crystalline limestone 

 and quartzite. Previous to this geographical name they were called azoic, metamor- 

 phic, or primary rocks. 



Granite is a word derived from the granular texture of the rock to which it is 

 applied. It is crystalline and composed of quartz, felspar, and mica. The felspar 

 usually gives the predominating color. When the granite is stratified, the laminae 

 being separated by thin scales of mica, it is gneiss or granite schist; if mica is in 

 excess, it is mica schist; when hornblende displaces the mica, it is syenite (named 



