Pirsson — Petrographie Province of Central Montana. 49 



Again, in the Castle district the stock at Blackhawk described 

 under the name of " diorite" has its central portion developed 

 as a monzonoid facies, as may readily be seen by reference 

 to its description and analysis.* In the Little Belts to the 

 north monzonite occurs in a considerable mass at Yogo Peak,f 

 and in the HighwoodsJ and Bearpaws§ it is a prominent type. 



Or again, shonkinitic facies of rock masses are found at Yogo 

 Peak in the Little Belts and in the stock at the head of Beaver 

 Creek in the Bearpaws, while in the Highwoods this rock is the 

 common type and found in numerous masses. 



So also rocks of tinguoid habit do not occur at all in Castle 

 Mountain or in the Little Belts in the southern part of the 

 province. They first begin to appear in the Highwoods in the 

 middle part; here they are rare, only a few occurrences being 

 noted, but in the Judith,] Little Eocky^f and Bearpaw** Moun- 

 tains which clefme the northern part of the province they are 

 very common rocks. 



• Other instances might be cited but these are sufficient to 

 indicate the idea involved. It is in intrusive rocks of various 

 kinds of occurrence, and perhaps more noticeably in dikes, that 

 this progression of types is seen. The extrusive rocks do not 

 occur so generally in this province that it may be observed 

 among them. 



It would be a matter of interest to know if this progression 

 of types is a peculiarity confined to this province and occasioned 

 by the local distribution of magmas or whether it is of more 

 general application. The writer has observed indications of 

 it in other places, as, for instance, in central New Hampshire, 

 where at Red Hill and Mount Belknap centers of alkalic mag- 

 mas occur. These are indicated at long distances by sporadic 

 dikes of bostonoid and camptonoid habits, the latter becoming 

 very numerous at the actual centers. But outside of the cen- 

 tra] Montana area the writer has not that intimate acquaintance 

 with other broad petrographic provinces which is necessary to 

 be able to apply this idea to them, and it must be left to others. 

 It would seem as if south Norway and central Italy and per- 

 haps the Bohemian Mittelgebirge, which is being so ably inves- 

 tigated by Hibsch, might afford good examples. 



Sheffield Scientific School of Yale University, 

 New Haven, Conn., December, 1904. 



* Bull. 139, p. 89. f Petrog. Little Belt Mts. p. 475. 



{Bull. 237, p. 76. § This Journal, vol. i, p. 355, 1896. 



1 18th Ann. Rep. U. S. Geol. Surv. Pt. iii, p. 566, 1898. 

 ^[Journal Geol., vol. iv. p. 419, 1896. 

 ** This Journal, vol. ii, p. 189, 1896. 



Am. Jour. Scl— Fourth Series, Vol. XX, No. 115.— July, 1905. 

 4 



