1914] Lawson: Is the Boulder " Batholith" a Laccolith ? 11 



Unfortunately Sales does not give us the details of his field obser- 

 vations on this important structural relation ; but his general state- 

 ments are borne out to some extent by Weed's map, 20 on which the 

 synclinal relation of the surrounding rocks to the central granitic 

 mass is well shown at the north end near Helena, at the south end 

 on the flanks of the Highland Mountains, and on the east side at the 

 south end of Bull Mountain. The general dip of the pre-granitic 

 formations towards the intrusive mass along its northern boundary is 

 more fully shown in Knopf's map. 27 



The directness of the genetic relationship of this general synclinal 

 structure to the intrusive act is, of course, debatable. It may be 

 claimed that the attitude of the rocks is but a manifestation of general 

 orogenic deformation and that a great pre-existant synclinal trough 

 was swallowed up in the batholith. But the intrusion of the Boulder 

 mass seems to have been closely associated in time with the orogenic 

 movements and the coincidence in time suggests a close genetic rela- 

 tionship. In his discussion of the Marysville intrusive mass Barrell 28 

 says : 



Toward the close of the Cretaceous or the opening of the Tertiary . . . 

 folded mountain structures arose, building great domes and arches. General up- 

 lift occurred, giving rise to plateau-like elevations. Volcanic forces awakened 

 throughout the entire Cordilleran region of America, producing eruptions, lava 

 floods, and granitic intrusions. . . . After the doming and marginal fault- 

 ing, the igneous activities of the Marysville district manifested themselves, 

 first as a widespread, rather meager injection of basic and intermediate dikes 

 and sheets. This was followed after an unknown interval, but while the 

 erosion surface was still some thousands of feet above the present level, by a 

 great granitic invasion, which gave rise to the Marysville batholith. This is 

 apparently but an insignificant outlier, however, of the far larger Boulder 

 batholith, which occupies over 2000 square miles immediately to the south. 

 The invasion of the batholith . . . produced much fracturing of the sur- 

 rounding rocks, resulting in many obscure faults bounding irregular crust 

 blocks from a few hundred to a few thousand feet across, the general effect 

 being to dome the cover upward at least 1000 and probably 2000 feet. 



It seems, therefore, that the deformation of the strata and the 

 invasion of the granite were in a broad sense coincident manifestations 

 of a general diastrophism and that the synclinal structure of the 

 rocks surrounding the Boulder mass is genetically connected with the 

 development of the latter. 



2«U. S. G. S. Prof. Paper 74, Plate 1. 

 27 U. S. G. S. Bull. 527, Plate 1. 

 2« Op. cit., p. 24. 



