4:6 Pirsson — Petvograpliic Province of Central Montana. 



This arrangement of magmas over the region is of course not 

 so well displayed as if there had been outbreaks of lava and 

 igneous intrusions in every ten square miles of it and all con- 

 forming to the rule, but it is believed by the writer that one 

 who reads carefully the facts previously stated and studies the 

 map must be struck with the disposition of the magmas about 

 a common center as shown in the mountain groups. It does 

 not appear as if this could be mere chance ; on the contrary, it 

 certainly seems to point to an orderly arrangement of things 

 according to some definite cause, whether we are able to discover 

 the latter or not. 



Bearing on Differentiation. — It will be noticed that this 

 arrangement is contrary to what obtains in most cases of local 

 differentiation such as those of Yogo and Bearpaw peaks and 

 Square Butte in this province, in which the border zones are 

 femic with a concentration of the salic components towards 

 the center. Washington* has shown that at Magnet Cove the 

 contrary is the case, the outer zone being more salic and the 

 inner part of strongly femic types. An instance of this also 

 occurs in this province in the " diorite " intrusion of Castle 

 Mountain near Blackhawk,f which at the center is a monzonoid 

 rock with 56 per cent of silica and grows steadily more siliceous 

 towards the periphery until it becomes a quartzose porphyritic 

 tj^pe. Other examples are described by BroggerJ in Norway 

 and Bamsay and Hackmann§ in Lapland. 



Washington in discussing these cases || is inclined to view 

 them as results of processes of solution and crystallization in 

 which the magma, composed of silica, alumina and alkalies, is 

 the solvent, the others being the solutes, and the solvent being 

 in excess tends to crystallize first at the outer margins. This 

 might explain such cases of local differentiation as are seen in 

 laccoliths like Square Butte, but it is clear that it could not be 

 applied to whole regions. For granting for the moment that a 

 parent body of homogeneous magma can form diverse smaller 

 bodies by some process, it could not do so over wide areas by 

 one of solidification ; the facts demand that the cleaved products 

 should remain liquid though these secondary bodies, after intru- 

 sion into place, might yield diverse products by crystallization. 

 The writer is not inclined to believe, on the other hand 1 , that 

 pure molecular flow, which Becker^ has shown must take 

 place with great and increasing slowness, can be a sufficient 



* Igneous Complex of Magnet Cove, Bull. Geol. Soc. Ainer., vol. xi, p. 407, 

 1900. 



fBull. 139, U. S. Geol. Surv., pp. 134 and 140, 1896. 



JBrogger : Zeit. f. Kryst., vol. xvi, 1890, p. 45. 



§ Bamsay and Hackmann : Fennia, vol. xi, No. 2, 1894. 



| Loc. cit., p. 408. 



■f This Journal, vol. iii. p. 21, 1897. 



