C— GEOLOGY. 45 



Mesozoic and older rocks, as was shown by Geikie, and has been borne 

 out by all subsequent observations, have emanated from a series of 

 fissures formed by uniformly directed tensional forces acting in the 

 earth's crust. Individual flows had no great thickness compared with 

 their great lateral extent, they were more or less unaccompanied by any 

 explosive action, and there was consequently little in the way of 

 associated pyroclastic deposits. They were erupted on a land-surface 

 and collectively made a covering that amounted to many thousands of 

 feet in thickness. Even now, after ages of denudation, thicknesses of 

 3,000 feet can be measured. 



They are the earliest and greatest expression of Tertiary igneous 

 activity and as such first claim our attention in view of their bearing on 

 later events. The similarity of type, widespread distribution, and great 

 aggregate thickness of the plateau lavas lead directly to the supposition 

 that beneath this north-west province lay an enormous intercrustal 

 reservoir filled with a basic magma that differed not at all in composition 

 from that of the lavas erupted at the surface. Further, this reservoir 

 must have been situated at such a depth that its minimum temperature 

 during the eruptive period was above the freezing point of any mineral 

 constituent of the magma. For all normal silicate magmas, differen- 

 tiation without crystallization is sufficiently improbable to enable us to 

 dismiss the separation of immiscible liquid fractions from our considera- 

 tions. Thus it would appear that the uniform composition of the lavas, 

 the general absence of phenocrysts of intrateliuric character and the 

 scarcity of cognate xenoliths may be taken as indications that crystalliza- 

 tion and consequent differentiation had not proceeded to any extent, and 

 that great depth and related elevated temperature were the restraining 

 factors. 



It does not concern us particularly what produced the tensional stress 

 in the crust with its accompanying fractures. It is sufficient to realise 

 that such a state of tension undoubtedly existed and that consequent 

 crustal weakness allowed large portions of the region to founder. The 

 causes have recently been discussed by Dr. J. W. Evans, who attributes 

 them to isostatic adjustment between the north-western continental area 

 and the Atlantic deeps. 



The foundering of the crust either as a whole, or more likely in restricted 

 but ever changing areas, produced the uprise of magma in the fissures and 

 the general outpouring of the plateau lavas ; and we may assume that the 

 end of this eruptive period coincided with temporarily restored equilibrium. 

 The next phase appears to me to be the local subsidence of the roof of the 

 deep-seated magma-basin. These local subsidences must have been more 

 pronounced than the general settling that was responsible for the extra- 

 vasation of the plateau lavas, and the magma, instead of merely rising up 

 more or less restricted fissures, ascended to take the place of the locally 

 subsiding crustal masses. The reason for this assumption is that within 

 the primary magma-basin differentiation was restrained by depth and 

 elevated temperature. Further, the initial temperature would not be 

 reduced to any extent by the outpouring of the lavas. Therefore, for 

 differentiation to take place, portions of the original magma would have 

 to occupy reservoirs of a local character situated in a zone of the earth's 



