REPORT OF TEE CHIEF ASTRONOMER 113 



SESSIONAL PAPER No. 25a 



PALEOZOIC LIMESTONES OF THE MACDONALD EANGE. 



DESCRIPTION. 



With reference to the fault troughs respectively occupied by the Kootenay 

 Tiver (at Gateway) and the Flathead river, the Galton-MacDonald 

 mountain system is a compound horst. We have seen that the Devonian 

 limestone at Tobacco Plains now stands at a common level with strata as old 

 as the base of the Siyeh. Much greater displacements at the western side of 

 the Flathead trough have dropped Devonian and Mississippian limestones down 

 into contact with the oldest members of the Galton series, including the Altyn 

 formation. The result of this faulting is peculiar, since a long, slab-like block 

 of Altyn, Hefty, and MacDonald beds is bounded on both sides by Mississippian 

 limestone. The younger, fossiliferous limestones form two masses separated by 

 the slab and may be referred to as the Western and Eastern blocks. 



The western block is well exposed only at comparatively few points; else- 

 where it is covered by heavy forest. The bounding faults are, therefore, mapped 

 only approximately. This limestone is dark bluish-gray, weathering light gray 

 to whitish. It is massive, rarely showing stratification planes; fetid under 

 the hammer; semi-crystalline, with the larger calcite crystals blackened by 

 films of bituminous matter. In one shear-zone the normal colour is changed 

 to yellowish gray or brown. At other outcrops the fresh limestone is crystal- 

 line and white; the bituminous matter has there been distilled out. 



The rarity of visible bedding-planes makes it impossible to make certain 

 as to the attitudes assumed by the limestone throughout the block. The best 

 exposures along the Commission trail, where it threads the canyon at the 

 Boundary line, show a horizontal position, but farther to the northwest probable 

 dips of about 30° to the southwest were observed. It is likely that the western 

 block is compound and bears numerous local faults and shear-zones. 



A few fossils were found at a cascade just south of the Commission trail 

 at 114° 38' W. Long., and 400 yards south of the Boundary slash (Station No. 

 1278). Dr. Ami identified these as including a species of Menophyllum and 

 an Athyroid form. The rock elsewhere bears crinoid stems. The horizon could 

 not be determined but it is ' presumably upper Mississippian.' 



The eastern block is composed of both Devonian and Mississippian lime- 

 stones which are greatly broken by step-faults. In the field no lithological 

 distinction could be made between the two limestones. Wherever fossils 

 occurred the rock was massive, crinoidal, bituminous, and gray, corresponding 

 in all respects to staple phases of the western block and of the limestone at 

 Tobacco Plains. The proved Devonian beds, however, are specially rich in 

 cherty nodules and are often mottled with irregular magnesian and dolomitic 

 parts. 



At the edge of the Flathead valley drift cover, 5,000-foot contour, and 

 1,000 yards north of the Boundary line, some 500 feet of unfossiliferous, pinkish- 

 gray, sandy beds were noted. These are generally magnesian and include thin 

 len~es of grit containing small, black pebbles of argillite. Cross-bedding was 



25a— vol. ii— 8 



