CHAPTEE II. 



VEGETATION OF THE LAUEENTIAK AND EAELT PALEO- 

 ZOIC — QUESTIONS AS TO ALGE. 



Oldest of all the formations known to geologists, and 

 representing perhaps the earliest rocks produced after our 

 earth had ceased to be a molten mass, are the hard, crys- 

 talline, and much-contorted rocks named by the late Sir 

 W. E. Logan Laurentian, and which are largely developed 

 in the northern parts of North America and Europe, and 

 I in many other regions. So numerous and extensive, in- 

 Ideed, are the exposures of these rocks, that we have good 

 reason to believe that they underlie all the other forma- 

 tions of our continents, and are even world-wide in their 

 distribution. In the lower part of this great system of 

 rocks which, in some places at least, is thirty thousand 

 feet in thickness, we find no traces of the existence of 

 any living thing on the earth. But, in the middle por- 

 tion of the Laurentian, rocks are found which indicate 

 that there were already land and water, and that the waters 

 and possibly the land were already tenanted by living 

 beings. The great beds of limestone which exist in this 

 part of the system furnish one indication of this. In the 

 later geological formations the limestones are mostly or- 

 ganic — that is, they consist of accumulated remains of 

 shells, corals, and other hard parts of marine animals, 

 which are composed of calcium carbonate, which the ani- 

 mals obtain directly from their food, and indirectly from 

 the calcareous matter dissolved in the sea- water. In like 



