Pirsson — Petrographic Province of Centred Montana. 41 



Consanguinity shown in textural habit. 



In some cases the consanguinity of the rock family is shown 

 in the repetition of certain textural habits. Thus the pseudo- 

 leucite basalts of both the Highwoods and the Bearpaws 

 closely resemble each other and both of them differ in habit 

 from the leucitic rocks of other regions, from those of Italy 

 for example. A most marked instance is also seen in the 

 minettes of High wood type (phyro-biotitic-cascadose). These 

 occur not only in the Highwoods but thirty miles to the north- 

 east on the Missouri River Mr. Weed collected similar rocks 

 and they occur doubtless in the Bearpaw Mountains.* One of 

 these from the Missouri River so exactly resembles the occurrence 

 on Williams Creek on the south slope of the Highwoods and 

 described in the memoir on the Highwood rocksf that hand 

 specimens of the two cannot be distinguished from one 

 another. So too, while each occurrence of shonkinite in the 

 region, in the Little Belts, the Highwoods and the Bearpaw 

 has its own peculiarities, yet taken together they form in sum 

 total a well marked family group. 



In the salic, feldspathic types, on account of their simpler 

 composition these evidences of family relationship are less 

 distinctly marked, and yet in the porphyries composing the 

 laccoliths in all the groups of the province, there appears to be 

 a tendency towards the repetition of a type with a certain tex- 

 tural habit difficult to describe but easily recognizable. It 

 appears to be largely conditioned by a certain abundance, size 

 and disposition of phenocrysts. There are many wide excep- 

 tions and variations of this, nevertheless the rule holds. 



Chemical evidences of consanguinity. 

 The strongest evidences which show that the rocks of these 

 various groups belong to a common family are to be found in 

 comparing their chemical compositions. For this purpose a 

 sufficient number of analyses are available for the Castle, Little 

 Belt, and Highwood Mountains. For the Bearpaws there are 

 enough to show the general character of the magmas, though 

 more would be desirable. For the Judith and Little Rocky 

 Mountains there is only one for each, but the general similarity 

 of the types, shown by their petrographic study, is sufficient to 

 indicate that they must agree in essential chemical characters, and 

 as the rocks are of very similar nature and of simple types 

 the two analyses must supplement each other fairly well. 

 The Moccasin Mountains, which are two compound laccoliths, 

 are practically a part of the Judith Mountains, and their rocks, 



* Judging from Dawson's description (op. cit., p. 46) it seems probable 

 tbat the same type also occurs in the Sweet Grass Hills. 



f Igneous Eocks of the Highwood Mts., Bull. No. 237, U. S. Geol. Surv., 

 1904, p. 142. 



