1 10 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



habit and most lithological details the dominant phase of the Eoosville is very 

 similar to that dominant phase of the Gateway formation. It seems, however, 

 that casts of salt-crystals are wanting in the younger formation. The metar- 

 gillite is composed of angular quartz and feldspar grains (averaging only 0-02 

 mm. in diameter) in an abundant matrix of sericite, chlorite, iron ore, and 

 possibly, in some beds, a little of the original argillaceous matter. The feldspars 

 again include orthoclase, microperthite, and plagioclase. Bedding planes are 

 well marked by glinting sericite in the form of innumerable minute foils and 

 shreds. 



Thus, at the top of the Galton series as at the bottom, static metamor- 

 phism has effectually changed the original clayey sediments into nearly or quite 

 holocrystalline rocks. The mica foils developed in the Hefty or MacDonald 

 strata are, at many horizons, larger than the micas characteristic of the Eoos- 

 ville, Phillips, or Gateway, and the top members of the series may have retained 

 , a greater quantity of original argillaceous matter. In these two respects the 

 older formations have, through deeper burial, suffered a slightly more advanced 

 metamorphism than the beds lying seven to ten thousand feet higher in the 

 series. Nevertheless, the evidence is clear that the Eoosville formation, like 

 the Kintla of the Lewis series, has been buried beneath many thousands of 

 feet of still younger strata, doubtless including the heavy Devonian and Car- 

 boniferous limestones; to that ancient burial the development of the metargilli- 

 tic facies of the Eoosville beds is due. 



The specific gravity of a type specimen from the metargillite is 2-730. 

 A somewhat weathered hand-specimen gave 2-675. The average for the forma- 

 tion is probably about 2-710. 



The Eoosville has yielded no fossils. The formation appears to be younger 

 than any beds belonging to the Lewis series as above described. It may prove 

 to be equivalent to an upper division of the Kintla which is not exposed in the 

 Boundary belt, or may represent the westward extension of a distinct forma- 

 tion. 



DEVONIAN FOEMATION IN THE GALTON EANGE. 

 DESCRIPTION. 



At the eastern edge of the drift-covered Tobacco Plains (115° 

 3' W. Long.), a block of fossiliferous Devonian limestone has been faulted 

 down into contact with the Gateway formation. On the west and south the 

 limestone is covered by drift and alluvium. The main fault which limits the 

 block on the east can be rather sharply located, the strikes of the limestone 

 and Gateway metargillite being nearly at right angles to each other. This fault 

 is marked on the map sheet, where it will be seen to run roughly parallel to 

 other faults that are responsible for the local graben character of the Eocky 

 Mountain Trench. The limestone is itself affected by numerous minor slips, 

 so that it is impossible to be certain of the thickness. In general, the block 



