22 THE STORY OF THE EARTH AND MAN. 



to remain. It seems also quite certain, and tliis 

 is a most important point for our sketch, that the 

 Laurentian ocean was not universal, but that there 

 were already elevated portions of the crust capable 

 of yielding sediment to the sea. 



In North America these Laurentian rocks attain 

 CO an enormous thickness. This has been estimated 

 by Sir W. E. Logan at 30,000 feet, so that the beds 

 would, if piled on each other horizontally, be as high 

 as the highest mountains on earth. They appear to 

 consist of two great series, the Lower and LTpper 

 Laurentian. Even if we suppose that in the earlier 

 stages of the world's history erosion and deposition 

 were somewhat more rapid than at present, the 

 formation of such deposits, probably more widely 

 spread than any that succeeded them, must have 

 required an enormous length of time. 



Geologists long looked in vain for evidences of life 

 in the Laurentian period ; but just as astronomers 

 have suspected the existence of unknown planets 

 from the perturbations due to their attraction, geolo- 

 gists have guessed that there must have been some 

 living things on earth even at this early time. Dana 

 and Sterry Hunt especially have committed them- 

 selves to such speculations. The reasons for this 

 belief may be stated thus: (1.) In later^ formations 

 limestone is usually an organic rock, produced by the 

 accumulation of shells, corals, and similar calcareous 

 organisms in the sea, and there are enormous lime- 

 stones in the Laurentian, constituting regular beds. 



