;240 DEPARTMENT OF THE INTERIOR 



2 GEORGH V., A. 1912 



Evidently neither of these two arguments is quite conclusive, but the 

 balance of probabilities is certainly on the side of the belief that the strata cut 

 by the Purcell sills lay nearly horizontal as the thick bodies were injected. In 

 view of the perfect conformity of the Moyie and Kitchener formations (both 

 laid down in shallow water) it appears probable that the surface of the Kitch- 

 ener formation was not elevated through the full 2,000 feet represented in the 

 total thickness of the Moyie sills; it seems more likely that the beds underlying 

 the sill-horizon were down-warped nearly or quite 2,000 feet, so a? to make room 

 'for the sill magma. 



Superfusion of Sill Magma. — The hypothesis carries the second assumption 

 'that the gabbroid magma was, at the time of intrusion, hot enough and fluid 

 enough to permit of the solution of a considerable body of quartzite and the 

 diffusion of the dissolved material to the upper contact. The assumption is 

 supported by the discovery of the great horizontal extent and uniform thick- 

 ness of the intrusive bodies; if the magmatic viscosity had been high, each 

 body would have probably assumed the true cushion sbape of the typical 

 laccolith. The extreme fluidity of the Purcell Lava is proved by the great 

 distances to which its flows ran before solidifying. If the Moyie and other sills 

 were but the contemporaneous intrusive facies of tbe same lava, the intrusive 

 magma must have been highly fluid. Its temperature was at least slightly 

 higher than that of the extrusive and therefore somewhat chilled lava; and, 

 secondly, the pressure of the few thousand feet of overlying Kitchener beds 

 could raise the solidifying point only to an insignificant extent (probably less 

 than 5° C). From a study of the grain in the Moyie sills, Lane has calculated 

 that the magma, when injected, must have been considerably superheated, and 

 therefore quite fluid.* 



Finally, whatever theory of the acid zones be adopted — whether that of 

 pure differentiation, of assimilation, or of both— the fact is clear, from the 

 foregoing lithological description, that the diffusion of silicious material 

 through the gabbro actually occurred on a large scale and that this diffusion 

 could not have taken place unless the original magma were possessed of a high 

 degree of fluidity. 



Chemical Comparison of Granite and Intruded Sediment. — A third, even 

 more clearly indispensable condition o| the - hypothesis relates to the composi- 

 tion of the invaded sediments. One of the most noteworthy features of the 

 huge series of conformable strata in the Creston-Kitchener series in this 

 particular district is the marvellous homogeneity of the whole group. As 

 already indicated, even the division into the two great subgroups, Creston and 

 Kitchener, is founded on merely subordinate details of composition. Hence 

 it is that the study of comparatively few type specimens can give a very 

 tolerable idea of the average constitution of the quartzites. For convenience 

 a brief description of both Creston and Kitchener specimens analyzed will be 

 here repeated. Single beds typical of the Creston occur interleaved in the 



' A. C. Lana, Jour. Canadian Mining Institute, Vol. 9, 1P06. 



