126 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



the variation of the fusion point of crystallizing minerals according to 

 the proportion of other minerals present, all of which forces would 

 have freer play near the surface, where the composition of the 

 magma would be less constant and the superincumbent pressure be 

 less. 



The contraction which these rocks have undergone must have taken 

 place at a period subsequent to their assumption of the solid state, and 

 would, therefore, naturally have an immensely less chance of exert- 

 ing a modifying influence upon the arrangement of their comjDonent 

 crystals than forces at work while it was yet plastic. The results of 

 such contraction are probably seen in the innumerable fissures that 

 traverse Laurentian rocks, and ai'e now filled with pegmatite granite 

 and other intrusive material. The fact that, as a rule, thei"e is an 

 exti'emely small amount of dislocation attending these fissures favours 

 the view that they are due to contraction of the solid rock rather 

 than to any kind o^ upheaval or similar disturbance. It is, more- 

 over, to be observed that the crumpling and folding of the original 

 surface of the earth, due to the expansion of solidification, would, by 

 the natural balance of physical forces, be prevented from extending to 

 excessive depths. The pressure due to the weight of the upper por- 

 tions of the globe's forming crust would at a certain depth counter- 

 balance the pressure of expansion, and long before this point was 

 reached the tendency to folding would be checked. The solidifying 

 magma would expand quietly and uniformly in all directions till 

 equilibrium was established, when, the resistance from above being- 

 equal to that in other directions, there would be no tendency to 

 diflferentiation in the arrangement of the crystals. The resulting rock 

 would be the granite, into which gneiss is often seen to graduate 

 when deniidation has exposed the lower portions. 



If we now pass on to consider the question of schistose cleavage, we 

 find that the instances cited show that such a cleavage may be 

 developed in igneous or intrusive rocks. This schistose structure 

 appears to be closely allied to gneissic foliation, though a rock may 

 be distinctly gneissic, and yet not at all schistose under the hammer. 

 Both are probably due, so far as dykes are concerned, to the same 

 causes. It is scarcely necessary to remark that there are two kinds 

 of cleavage, one due to a parallel arrangement of the foliated minerals 

 in the rock, produced very probably by the operation of the same 

 forces as those to which the foliation of gneiss is ascribed, and 



