50 THE LESSON OF EVOLUTION 



tion and a nomenclature, it will be universally adopted. 

 At present this is not the case, and in this essay I 

 have followed for the most part the Canadian autho- 

 rities, 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 conglomerates 

 called the Huronian system, which probably repre- 

 sents 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 Keweenewan 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 



