238 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



Below sill B is a 550-foot band of quartzite with strike N. 25° W. and dip 

 65° E.N.E. 



Sill G, about 530 feet thick, is well exposed in the Boundary-line section 

 as well as in that on the wagon road north-northwest of the summit of the 

 mountain, where three of the analyzed specimens were collected. This body is 

 the most striking of all in its evidence of gravitative differentiation. The 80- 

 foot zone of biotite granite at the top passes gradually into the underlying 

 110-foot zone of hornblende-biotite granite, which, in turn, merges into the 

 60-foot zone of intermediate rock overlying the 280-foot zone of the usual 

 Purcell gabbro at the bottom of the sill. 



Between sills C and D comes a band of quartzite with strike N. 20° W., 

 dip 60° E.N.E. 



Sill D is poorly exposed but seems to be largely composed of the usual 

 gabbro, overlain by successive zones of intermediate rock and hornblende- 

 biotite granite. The outcrops do not suffice to show the exact thickness of any 

 of these zones, but it seemed clear in the field that the total thickness of the 

 two more acid zones was little more than 100 feet. 



Between sills D and E is a band of quartzite, estimated as about 750 feet 

 in thickness; its strike is N. 30° W. and dip 60° E.N.E. 



Sill E. The lower part of sill E is well exposed a few hundred yards south 

 of the Boundary slash, but its contact with the overlying quartzite was nowhere 

 discovered. As already noted, the existence of that layer of quartzite was not 

 even suspected in 1904, as it was entirely covered by talus along the line of 

 traverse then followed by the writer. It then seemed most probable that the 

 gabbro masses exposed at the top and bottom of the great talus slope formed 

 parts of a single sill. Eor 100 feet or more from its lower contact the rock of 

 sill E is practically the usual gabbro of the Purcell sills. That zone is overlain 

 by a zone of intermediate rock, the top of which has not been discovered. The 

 two zones show a gradual transition into each other. 



ORIGIN OF THE ACID PHASES. 



Preferred Explanation. — Among all the conceived hypotheses as to the 

 origin of the acid zones, the writer has been forced to retain one as the best 

 qualified to elucidate the facts concerning the Moyie sills. More important 

 still, this hypothesis, better than any of the others, affords a coherent, fruitful, 

 and, it seems, satisfactory explanation of similar occurrences in other parts 

 of the world. It will be presented in some detail, since it is believed that these 

 sills, and similar ones in Minnesota and Ontario represent gigantic natural 

 experiments bearing on the genetic problem of granites and allied rocks in 

 general. The view adopted includes what has been called ' the assimilation- 

 differentiation theory.' The acid zone is thereby conceived as due to the diges- 

 tion and assimilation of the acid sediments, together with the segregation of 

 most of the assimilated material along the upper contact. 



