98 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



On the eastern edge of Tobacco Plains a smaller block of Devonian lime- 

 stone and dolomitic quartzite, estimated to show a thickness of 1,600 feet, has- 

 been faulted down into contact with the Gateway formation. Its relation to 

 the Galton series could not be directly observed. As elsewhere in this part of 

 the Cordillera the limestone doubtless passed gradually upwards into the 

 Mississippian limestone but no rock of that age was determined on the west 

 side of the range. 



Altyn Formation. 



The oldest formation exposed in the MacDonald range where crossed by 

 the Boundary belt consists of strata essentially similar in stratigraphic relations 

 and in composition to the uppermost member of the Altyn formation of the 

 Lewis series. The identity is so complete that the same name may well be 

 used for these rocks of the MacDonald range. Though not exposed in the 

 Galton range at the Boundary, the equivalents of the same strata unquestion- 

 ably underlie the surface rocks of that range as well. 



West of the Flathead the Altyn crops out at only two localities within 

 the Boundary belt. On the ridge overlooking the Flathead from the west and 

 culminating in Mt. Hefty, the triangulation peak, this formation forms part 

 of a block faulted into contact with Carboniferous limestone. Just south of 

 the Boundary monument on the ridge, a thickness of 650 feet was measured 

 for the Altyn but the base was not seen, the lower beds having been faulted 

 away, out of sight. The second locality is that at a box-canyon six miles due 

 west of the Hefty ridge; there, only 120 feet of the uppermost beds are exposed. 



The Altyn formation in the MacDonald range consists of a succession of 

 fairly homogeneous but very thin-bedded, silicious dolomites. The rock is 

 always compact and relatively hard, yet very fissile on account of the thin 

 bedding. The layers vary from 1 cm. to 10 cm. in thickness. When fresh it 

 is slightly gray or greenish gray; it weathers buff and bright brownish yellow. 

 Quite subordinate are more massive beds (three to five feet thick) of gray, 

 calcareous quartzite. Towards the top, thin intercalations of red calcareous 

 sandstone and argillite indicate transition to the Hefty formation above. Cer- 

 tain of the magnesian limestone beds are somewhat argillaceous and then 

 commonly bear sun-cracks. 



Microscopic and chemical analysis of the dominant rock, the silicious 

 dolomite, shows that it is similar to the principal phase of the upper Altyn in 

 the Lewis and Clarke ranges. The main mass of the rock is a very compact 

 carbonate, occurring in grains from 0-005 mm. to 0-03 mm. Angular particles 

 of quartz, averaging 0-02 mm. in diameter, are accessory constituents which, 

 in certain layers, may become quite abundant. This quartz was probably in 

 part of clastic origin but some of it may have been due to the recrystallization 

 of colloidal silica. When the carbonate is in contact with quartz, the former 

 often shows clean-cut, rhombohedral outlines. 



