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CHAPTER V. 



LAURENTIAN, CAMBRIAN, AND LOWER SILURIAN 

 ROCKS. 



THE LAURENTIAN ROCKS are the oldest formations at 

 present known in the world. They are metamorphic 

 and mostly gneissic in character, and were for long 

 classed as granitic and igneous rocks till their true 

 nature was shown by the late Sir William Logan. 

 They occupy vast tracts of country in Labrador and 

 Canada, and are well seen on the north of the river 

 Ottawa. They consist of two divisions, Lower and 

 Upper Laurentian^ the upper, according to Logan, 

 lying quite unconformably on the lower strata. The 

 gneiss of the lower division is chiefly orthoclase gneiss 

 of great thickness, and it is inter stratified with several 

 thick bands of crystalline limestone, sometimes ser- 

 pentinous, in one of which a remarkable foraminifer 

 called Eozoon Canadense was found. This is the 

 oldest known fossil. The upper Laurentian rocks, 

 which also contain beds of limestone, are to a great 

 extent formed of Labrador felspar, and in these no 

 fossils are known. 



In the Outer Hebrides and on the west coast of the 

 Highlands between Cape Wrath and Tiree, Laurentian 

 rocks occur of highly metamorphic gneiss, interpene- 



