GNEISSIC FOLIATION. 125 



ticulavly those usually referred to as granitoid gneiss. Tlie crunip- 

 ng and folding of the archean rocks is generally ascribed in text- 

 books to a process of contraction, due to the cooling of the earth's 

 crust or to the contraction of the interior molten globe, and the 

 consequent collapse upon it of the less rapidly cooling crust. This 

 explanation is probably the true one to account for the folding of 

 post-archean strata, and much of the disturbance that aflects the 

 archeans themselves is doubtless due to the operation of such forces. 

 But if it be admitted that the Laurentian gneisses ever so far resem- 

 bled intrusive gneiss as to have been in the condition of a molten 

 magma, then we have a prior and more important force to take into 

 consideration, viz., the expansion of solidification at the time when 

 the i-ock was passing from the fluid to the crystalline state. Such 

 an expansion would be amply sufficient to throw the solidifying but 

 yet plastic rock into the violent folds and contortions in which we 

 to-day find the fundamental gneiss wherever it is exposed the world 

 over. The same expansive pressm-e would, as in the case of the 

 dykes, have a decided tendency to arrange the precipitated crystals 

 in a more or less definite way with their longest diameters parallel 

 to the line of least resistance, i.e. parallel to the axes, whether straight 

 or curvilinear, of the folds. This tendency to gneissic arrangement 

 would be aided by the pulling and flow that would necessarily be 

 induced by the folding of such a partially crystalline still plastic 

 mass. This seems to me to be the most plausible hypothesis to 

 account for the origin of tlie granitoid gneisses of the Laurentian, 

 and it is certainly more in harmony with the phenomena we find in' 

 the archean field than a theory which holds that they are the altered 

 remains of once aqueous sediments. The marvellously intricate con- 

 tortions and intermingling of the gneiss seen not only on natural 

 exposures in the Lake of the Woods region, but also in some railway 

 cuttings between Port Arthur and Winnipeg, leave room for no 

 other conception than that the gneiss must have been in a perfectly 

 viscid condition. It is not at all probable that the explanation 

 advanced to account for the origin of the granitoid gneisses is true 

 of the more finely laminated and bedded gneisses that appear to 

 occupy a higher position in the Laurentian system. This lamina- 

 tion can, without serious objection, be ascribed to regularity of 

 crystalline precipitation, a stratiform arrangement due to difierence 

 of specific gravity in different portions of the original magma, and to 



