Pirsson — Petrograjphic Province of Centrdl Montana. 35 



Art. Y. — The Petrographic Province of Central Montana ; 

 by L. Y. Pirsson. 



Introduction. 



Definition of the province. 



Consanguinity shown by minerals. 



Augite. 



Biotite. 



Hornblende. 



Feldspars. 



Absence of minerals. 

 Consanguinity shown in textural habit. 

 Chemical evidence of consanguinity. 



General law of the province. 



Application to the region. 

 Geographical arrangement of magmas. 



Bearing on differentiation. 

 Regional progression of types, 



Introduction. 



The fact that in certain areas of the world's surface the 

 igneous rocks have common characteristics, which serve to 

 ally them together and to define them from the rocks of other 

 areas, is now well recognized by petrographers. These com- 

 mon features are sometimes expressed in the minerals, some- 

 times in the chemical composition of the magmas and some- 

 times in peculiarities of texture, but usually in a union of 

 these qualities. In some cases these features are clearly 

 marked, in others they are but slightly developed ; neverthe- 

 less, like those indescribable characters which define a man as 

 belonging to one nation rather than to another, they are easily 

 recognized by the experienced eye. 



The formulation of this principle, that the rocks of a given 

 region may be thus genetically related, we owe to Judd^* and 

 it has since been elaborated and applied with fruitful results 

 to various regions by Iddings,f who developed it under the 

 expression " con sang ncinity of igneous rocks." Since then the 

 idea has been applied to various regions by other petrograph- 

 ers ; so, for example, Lacroix in a recent very interesting memoir 

 on the alkalic rocks of northwest Madagascar, calls attention 

 to the great belt of types rich in soda that stretches along the 

 eastern coast of Africa.;); Of all the various areas, however, 

 where the consanguinity of igneous rocks has been studied and 

 these relationships pointed out, there is probably none better 

 known or more thoroughly investigated than that of South 



*Quar. Jour. Geol. Soc, 1886, vol. xlii. p. 54. 



f Origin of Igneous Eocks. Bull. Phil. Soc. Washington, xii, p. 128, 

 1892. 



£Koches alcaline de Prov. Petrograph. d'Ampasindava Nouv. Arch. d. 

 Museum, 4 me Ser., vols, i et v, 1902, 1903. 



