16 



THE DAWN OF LIFE. 



must be, unknown to us ; for it was only after the 

 Laurentian rocks liad been deposited, and after tbe 

 shrinkage of the earth's crust in subsequent times 

 had bent and contorted them, that the foundations 

 of the continents were laid. The rude sketch map 

 of America given in fig. 4 will show this, and will 

 also show that the old Laurentian mountains mark 

 out the future form of the American continent. 



Tig. 4. The Laurentian Nucleus of the American Continent, 



Rocks so highly altered as the Laurentian beds can 

 scarcely be expected to hold well characterized fossil 

 remains, and those geologists who entertained any 

 hope that such remains might have been iDreserved, 



