THE HISTORY OF THE NORTH ATLANTIC 69 



not suppose that the old gneiss is an ordinary sediment, but 

 that all regard it as formed in exceptional circumstances, these 

 circumstances being the absence of land and of subaerial 

 decay of rock, and the presence wholly or principally of the 

 material of the upper surface of the recently hardened crust. 

 This being granted, the question arises, Ought we not to com- 

 bine the several theories as to the origin of gneiss, and to 

 believe that the cooling crust has hardened in successive layers 

 from without inward; that at the same time fissures were 

 locally discharging igneous matter to the surface ; that matter 

 held in supension in the ocean and matter held in solution by 

 heated waters rising from beneath the outer crust were ming- 

 ling their materials in the deposits of the primitive ocean ? 1 

 It would seem that the combination of all these agencies may 

 safely be evoked as causes of the pre-Atlantic deposits. This 

 is the eclectic position I have maintained in a previous chap- 

 ter, and which I hold to be in every way the most probable. 



Let us suppose, then, the floor of old ocean covered with 

 a flat pavement of gneiss, or of that material which is now 

 gneiss, the next question is, How and when did this original 

 bed become converted into sea and land ? Here we have some 

 things certain, others most debateable. That the cooling 

 mass, especially if it was sending out volumes of softened 

 rocky material, either in the form of volcanic ejections or in 

 that of matter dissolved in heated water, and piling this on the 

 surface, must soon become too small for its shell, is apparent ; 

 but when and where would the collapse, crushing and wrink- 

 ling inevitable from this cause begin? The date is indi- 

 cated by the lines of old mountain chains which traverse the 

 Laurentian districts; but the reason why is less apparent. 

 The more or less unequal cooling, hardening and conductive 

 power of the outer crust we may readily assume. The drifthge 

 unequally of water-borne detritus to the south-west by the 

 1 Hunt, Transactions Royal Society of Canada, 1885. 



