Ill] THE OLDEST ROCKS. 35 



wide-spread metamorphism, and to be made up of post-archaean 

 rocks. 



The first formed rocks no doubt became at once the prey of 

 denudation and disintegration, and on their surface would be 

 accumulated the products of their own destruction ; newer strata 

 would entirely cover up portions of the original land, to be in 

 their turn succeeded by still later deposits. There is reason to 

 believe that in the remotest ages of the Earth's history, the 

 forces of denudation and igneous activity were more potent 

 than in later times, and thus the oldest rocks could hardly 

 retain their original structure through the long ages of 

 geologic time. The earliest representatives of organic life were 

 doubtless of such a perishable nature that their remains could 

 not be preserved in a fossil state even under the most favour- 

 able conditions. Such organisms, whether plants or animals, 

 as possessed any resistant tissues or hard skeletons might 

 be preserved in the oldest rocks, but as these strata became 

 involved in earth-foldings or were penetrated by injections of 

 igneous eruptions, the relics of life would be entirely destroyed. 

 It is, in short, practically hopeless to look for any fragments of 

 the primitive crust except such as have undergone very con- 

 siderable metamorphism, and equally futile to search for any 

 recognisable remains of primitive life. 



In many parts of the world vast thicknesses of rock 

 •occur below the oldest known fossiliferous strata ; these consist 

 largely of laminated crystalline masses composed of quartz, 

 felspar, and other minerals, having in fact the same composition 

 as granite, but differing in the regular arrangement of the 

 constituent parts. To such rocks the terms gneiss and schist 

 have been applied. Rocks of this kind are by no means always 

 ■of Archaean age, but many of the earliest known rocks consist 

 of gneisses of various kinds, associated with altered lavas, 

 metamorphosed ashes, breccias and other products of volcanic 

 activity ; with these there may be limestones, shales, sandstones, 

 and other strata more or less closely resembling sedimentary 

 deposits. Such a succession of gneissic rocks has been described 

 as occupying a wide area in the basin of the St Lawrence river, 

 and to these enormously thick and widespread masses a late 



3—2 



