REPORT OF THE CHIEF ASTRONOMER 117 



SESSIONAL PAPER No. 25a 



14. Cliothyridina hirsuta. Specimens large and lauiellose. Ragged 

 edge of lamellae like spines in previously described specimens. 

 See also Athyris hirsuta, Hall; figs. 18-21 (Spergen Hill), and 

 PI. 6, Vol. 1, Bull. No. 3, Amer. Museum of Natural History. 



Station No. 1287—Thvee hundred yards west of station No. 1285; 5,800- 

 foot contour. 



Age: Probably upper Mississippian. 

 Genera and species: 



1. Lithostrotion, sp. 



2. Syringopora, sp. 



3. Menopliyllum, sp. 



STRUCTURE OF THE GALTON- MACDONALD MOUNTAIN SYSTEM. 



The geosynclinal rocks between the Flathead and the Rocky Mountain 

 Trench at the Forty-ninth Parallel are very much more deformed than are those 

 of the Clarke range or the Lewis range. The exceedingly inflexible nature of the 

 rocks has prevented the development of systematic folds; the structure all across 

 the Galton-MacDonald system is almost entirely determined by faulting. At 

 least twelve major fault-blocks are represented in the map sheets as occurring 

 in the five-mile belt where it crosses the two ranges. Within the belt the dips 

 range from 0° to 90°, averaging about 30°. 



The most easterly and the most westerly blocks contain, respectively, Car- 

 boniferous and Devonian limestones which, excepting the Kishenehn lake beds, 

 are the youngest bed-rock formations in the Rocky Mountain system at this 

 latitude. These particular blocks are of special interest since they clearly 

 show the magnitude of the displacements to which the Flathead trough and 

 the Rocky Mountain Trench owe their origin. The Carboniferous limestone 

 on the west side of the Flathead is on the same level with strata on the east 

 side, belonging to the lower Appekunny. One may fairly estimate that a net 

 displacement of at least 15,000 feet or possibly 20,000 feet is here indicated. 

 The western part of the Clarke range has been lifted nearly or quite three 

 miles higher than the most easterly block of the MacDonald range. The latter 

 block is downthrown by an even greater amount with respect to the block next 

 on the west. The Carboniferous limestone at the Flathead valley is, in fact, 

 the visible upper portion of a broad block or series of parallel blocks which have 

 been dropped a minimum of about three miles below the adjacent blocks of the 

 Clarke and* MacDonald ranges. The Flathead trough is thus structurally a 

 typical fault-trough or * graben.' It is also highly probable that the depression 

 has always been a graben in a topographic sense. It has been partially filled 

 with lake beds and has been deformed by the folding of those beds but there is 

 no evidence that the initial trough form was ever quite destroyed. 



It will be observed from the map sheet that the Carboniferous limestone of 

 the MacDonald range occurs in two different fault-blocks separated by a 



