LAURENTIAN SYSTEM. 87 



tend to obstruct rather than facilitate. It is to the oldest 

 of these so-called systems that we now direct attention ; to 

 the conditions under which they seem to have been de- 

 posited; and to the kind of life that appears to have 

 peopled the lands and waters of their respective periods. 

 We have classed them as the " Primary Periods/' because 

 there is really a greater similarity between their rocks and 

 fossils than there is between the rocks and fossils of any 

 subsequent periods ; because, so far as we know, their strata 

 are all truly marine deposits; and further, because their 

 fossil forms belong (speaking in general terms) to inverte- 

 brate types zoophytes, shell-fish, Crustacea, &c. and are 

 specially characterised by the absence of the vertebrates 

 the fishes, reptiles, birds, and mammals. 



Beginning with the LAURENTIAN, which term has been 

 employed by Sir William Logan of the Canadian Geological 

 Survey, to designate the highly crystalline strata which 

 belong especially to the valley of the St Lawrence, and 

 constitute the great bulk of the Laurentide mountains, we 

 may take Sir William's own description of the rocks which 

 compose this oldest and deepest of sedimentary formations. 

 " The rocks of this system," he says, " are almost without 

 exception ancient sedimentary strata which have become 

 highly crystalline. They have been very much disturbed, 

 and form ranges of hills having a direction nearly north-east 

 and south-west, rising to the height of 2000 or 3000 feet, 

 and even higher. The rocks of this formation are the most 

 ancient known on the American continent, and correspond 

 probably to the oldest gneiss of Finland and Scandinavia, 

 and to some similar rocks in the north of Scotland. They 

 consist, in great part, of crystalline schists (chiefly gneissoid 

 or hornblende), associated with felspars, quartzites, and lime- 

 stones, and are largely broken up by granites, syenites, and 



