i$ Pirsson — Petrographie Province of Central Montana. 



data in hand, to solve the problem of general differentiation 

 over wide regions must be a mere speculation. While the 

 physical chemist therefore is attacking the problem on one side, 

 it remains for the petrographer, on the other, to gather data 

 regarding the occurrences of igneous rocks and their interrela- 

 tionships and characters over definite areas, and the present 

 article can be considered as a contribution towards this end. 



The Regional Progression of Types. 



It is desired to call attention here to a phase of the occurrence 

 of rock types in the province in the hope that petrographers 

 may observe if it is of general application in different petro- 

 graphic provinces. 



In lieu of a better name it may be called the regional pro- 

 gression of types, and the idea involved in this term is as follows : 

 while in a given province there are certain family characters 

 serving to bind the various rock types into one clan, yet from 

 place to place within its limits the magmas may vary greatly 

 from each other, and there may be, as in the Montana prov- 

 ince, a number of centers with complexes of their own. It is 

 so to speak that the clan is made up of a number of families each 

 of which consists of individuals. In traversing the area from 

 one family to another, the observer will note that certain types 

 which are rare or sporadic in the first will become numerous 

 or even dominating ones in the second. Long before the second 

 family is reached its types begin to appear, and as the area is 

 approached they are likely to become more numerous, then 

 attain their greatest frequency and die away beyond. Thus 

 there is an overlapping of types and the rare one of a given cen- 

 ter of igneous rocks becomes the common one of a neighboring 

 center. It of course depends upon the gradual change in the 

 character of the magmas. 



Some instances of this which have been observed in this prov- 

 ince are as follows. In the Castle Mountains a single sporadic 

 case of a monchiquoid rock was observed.* Going northward 

 into the Little Belts they begin to be more common, and the 

 author has described them under the name of " analcite basalts,"f 

 while still farther to the northward in the High woods these are 

 exceedingly common rocks. They occur for the greater part 

 in dikes but in the Highwoods in flows also. In the midst of 

 the Castle rocks this type appeared out of place when consid- 

 ered only by itself, but if we consider it not as a member of 

 the Castle family, but of the central Montana clan, its occur- 

 rence falls into order. 



*Bull. 139 U. S. Geol. Surv., pp. 68 and 114, 1896. 



fPetrog. Little Belt Mts., 20th Ann. Rep. U. S. Geol. Surv., Pt. iii, p. 543, 

 1900. 



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