14 THE LESSON OF EVOLUTION 



so soon as the officers conducting the geological 

 surveys of Canada and the United States agree on a 

 classification and a nomenclature, it will be univer- 

 sally adopted. At present this is not the case, and 

 in this essay I have followed for the most part the 

 Canadian authorities, who first discovered these 

 rocks, and who have for many years devoted an 

 immense amount of labour to mapping them. 



The oldest rock system known to us is composed 

 chiefly of gneiss, sometimes passing into granite ; 

 and it probably represents the Azoic era. It is called 

 the Laurentian system. Above it, in discordant 

 sequence, is found in Canada a series of schists, 

 arkoses, quartzites, greywackes, and schistose con- 

 glomerates called the Huronian system, which pro- 

 bably represents the Protozoic era. However, in 

 order to avoid using theoretical names, which may 

 be incorrect, the Laurentian and Huronian are called 

 collectively the Archaean era. 



Immediately above the Huronian there is a great 

 unconformity, marking a considerable interval of 

 time ; and the succeeding rocks are called Keweene- 

 wan in Canada and Algonkian in the United States. 

 They are composed of a great thickness of sandstones 

 and slates sometimes locally altered into schists 

 which underlie the Cambrian system, the base of 

 which is marked by what is known as the Olenellus 

 fauna, from the occurrence in it of the trilobite called 

 Olenellus. Let us look at these more in detail. 



The Laurentian Period. The Laurentian in 

 Canada consists of two formations. The lower 

 known as the fundamental gneiss is of igneous 



