462 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



chronology, a near magmatic relationship for the two batholiths is indicated by 

 the essential similarities in the optical properties of the respective minerals. 

 These likenesses are observable in the quartz, microperthite, microcline, ortho- 

 clase, biotite, and the accessories, as well as in the hornblende which, as we have, 

 seen, is very rare in the Cathedral batholith. 



It would be a matter of the highest importance if one could demonstrate 

 the cause of this blood-relationship between the two batholiths. To say that 

 they are magmatic differentiates is only to restate the petrogenic problem. The 

 profitable questions are : What was differentiated in the two intrusive periods ; 

 and, what was the actual differentiating process? 



These questions cannot be answered with assurance. All that seems possible 

 now is to indicate the lines on which future investigation is needed. To do 

 even that would anticipate part of chapters XXVI. and XXVII. and the writer 

 will here offer only one conjecture as to the relation between the bodies. The 

 guess is based on the proved efficiency of density differences to explain splitting 

 in a heterogeneous magma, like that which composed the Moyie sills; secondly, 

 on the view that a mediosilicic magma tends to separate into the antagonistic 

 gabbroid (basaltic) and granitic magmas, this separation taking place with 

 special 'rapidity just before solidification of the original magma could take 

 place. 



Let us assume that part of the Similkameen granodiorite long remained 

 molten or was, by whatever means, partly remelted, and then gradually cooled. 

 It is conceivable that during the cooling the basic elements corresponding in 

 total composition to a gabbro, would settle down, leaving a persilicic residue in 

 the upper part of the magma chamber. To develop the hypothesis still further 

 the basic differentiate is assumed to have the composition of the local Ashnola 

 gabbro. Finally, it is assumed that just one-fifth by weight of the remelted 

 granodiorite settles out, this particular proportion being that which would give a 

 residue with silica very nearly equal to that in the Cathedral granite. The ; 

 residue has thus been calculated and found to be fairly close in composition to 

 the Cathedral granite in all the other essential oxides. The result of the cal- 

 culation is shown in Col. 3 of Table XXIX. Cols. 1, 2, and 4 respectively 

 state the analyses of the Ashnola gabbro, the Similkameen granodiorite, and the 

 Cathedral granite. 



