186 R. A. Daly — Secondary Origin of Certain Granites. 



This divergence of view is, of course, due to the lack of 

 definite knowledge of the vital conditions controlling the 

 activities of such an intrusive body as the Christiania granite. 

 The study of its accessible contacts can, of itself alone, furnish 

 neither proof nor disproof of the doctrine of wholesale assimi- 

 lation. Without the aid of other geological data the attempt 

 to solve the problem is like the attempt to produce graphically 

 a complex curve of which but two points are known and fixed. 

 Deep-seated assimilation about any magma chamber can only 

 be finally discussed and evaluated if the complete form of the 

 chamber and the complete composition of its rock-filling are at 

 least tolerably known. 



The present paper furnishes a brief discussion of a number 

 of cases where it is believed that magmatic assimilation on a com- 

 paratively large scale has taken place. It is believed, further, 

 that the geological conditions in these cases supply elements 

 generally untouched in earlier discussions of the doctrine. The 

 original magma had the composition of a gabbro intruded in 

 the manner of sills; the invaded formations are ancient sand- 

 stones, both normal and feldspathic, with associated argillites 

 or schists ; the invaded formation, in every case, is more acid 

 than the gabbro ; the product of assimilation is always a granite 

 graduating into granophyre. The acid magma is believed, 

 however, to have been derived indirectly from the compound 

 magma of assimilation through a systematic kind of differen- 

 tiation. The primary cause of the differentiation is referred 

 to the perfect or nearly perfect density stratification of each 

 magmatic chamber. 



The result of the investigation has been to confirm the 

 writer's general theoretical conclusions on the subject of assi- 

 milation where it was necessarily introduced among the tests 

 of the hypothesis of magmatic stoping*. Assimilation and 

 differentiation are not antagonistic processes ; both of them are 

 involved in the secondary origin of some granites. 



A. The Sills of the British Columbia (International) Boundary. 



During the field season of 1904 the writer developed a geo- 

 logical structure section along the 49th parallel of latitude 

 between Port Hill, Idaho, and Gateway, Montana, the two 

 points where the Kootenay River crosses the boundary line 

 between Canada and the United States. It was found that 

 the mountains traversed by the section are for the most part 

 composed of two very thick siliceous sedimentary formations 

 which, in all probability, are of pre-Cambrian age. The two 

 are conformable. 



The lower formation has been called the Creston quartzite. 

 It is a remarkably homogeneous, highly indurated light- to 

 medium-gray sandstone, generally thick-platy in structure but 

 * This Journal, xv, 269, 1903, and xvi, 107, 1903. 



