REPORT OF THE CHIEF ASTRONOMER 57 



SESSIONAL PAPER No. 25a 



The total thickness of the Altyn as shown in this Oil creek section is 

 much greater than that seen farther south by Willis (1,400 feet). The difference 

 is not to be explained by overfolding or overthrusting. Individual beds and 

 groups of beds are, it is true, considerably crumpled, especially in the lower 

 part of the section; but the average southwesterly dip of about 30° is preserved 

 throughout. The evidence of original conformity from top to bottom seems 

 as clear. 



Lit'aologically, the sediments here differ from those described by Willis 

 in carrying a notable proportion of rounded grains of quartz and feldspar. 

 The sandy and gritty strata occur chiefly in the middle of the section, there 

 totalling nearly 1,000 feet in thickness. It is thus convenient to recognize a 

 tripartite division of the Altyn as exposed along the International Boundary :— 

 An upper member (a and b) of thin-bedded, silicious dolomite 1,250 feet 

 thick; a middle member of thick-bedded, massive arenaceous dolomite and 

 calcareo-magnesian sandstones (c, d and e), 1,350 feet thick; and a lowest mem- 

 ber of generally thin-bedded, silicious dolomite (f and g), at least 900 feet 

 thick, containing sandy beds toward the bare. Nowhere in the formation were 

 there found sun-cracks, rill-marks, ripple-marks or any other indication that 

 the sediments were laid down in very shallow water or on a bottom laid bare 

 between tides. 



A visit was paid to Chief mountain and to the original locality at Altyn, 

 Montana, where the rocks were found to correspond to Willis' description 

 except in being often distinctly arenaceous. Willis' brief summary of the facts 

 observed by him reads as follows : — 



'Limestone of which two members are distinguished; an upper mem- 

 ber of argillaceous, ferruginous limestone, yellow, terra-cotta, brown, and 

 garnet red, very thin-bedded; thickness, about 600 feet; well exposed in 

 summit of Chief mountain; and a lower member of massive limestone, 

 grayish blue, heavy-be Jded, somewhat siliciou 5 , with many flattened con- 

 cretions, rarely but definitely fossiliferous; thickness, about 800 feet; type 

 locality, basal cliffs of Appekunny mountains, north of Altyn, Swift 

 Current valley.'" 



As the formation is followed southeastward the uppermo3t member shows 

 a decided darkening of tint — to terra-cotta, red, and brown of various deep 

 shades, which then dominate the lighter buff colour characteristic of that mem- 

 ber at the Boundary. It seems clear that the whole of the lowest member and 

 part of the middle member of the Altyn at the Boundary are not exposed in 

 the sections studied by Willis. On the whole the field relations in the Oil 

 creek section are more favourable to giving one an accurate idea of the whole 

 Altyn formation than are the field relations at either Chief mountain or at 

 Altyn itself. 



As already noted, this great formation is heterogeneous but every bed of 

 it seems to carry a notable percentage of carbonates. The cement of even the 



*Bull. Geol. Soc. America, Vol. 13, 1902, p. 317. 



