REPORT OF THE CHIEF ASTRONOMER 275 



SESSIONAL PAPER No. 25a 



Area West of Salmon River. — Dark greenish, or dark gray to black phyllite, 

 alternating with blackish quartzite, is the dominant rock on both banks of the 

 Pend D'Oreille, from its confluence with the Salmon to its mouth at Waneta. 

 The schists enclose lenses of white to gray marble, varying from ten feet or 

 less to 200 feet or more in thickness. Near the Columbia and especially on 

 the west side of that river, the phyllites and quartzites are associated with 

 very abundant, thick masses of greenstone and altered basic breccias, so that 

 it there becomes very difficult to separate the Pend D'Oreille group from the 

 younger Rossland Volcanic group. 



Lithologically, there is a great similarity between the respectively dominant 

 rock types on both sides of the Salmon but it is also clearly impossible to 

 develop a useful columnar section of these metamorphosed sediments along the 

 lower Pend D'Oreille. The great limestone is not represented. It is, however, 

 probable that most of the phyllite and quartzite is the equivalent of the 

 rocks tentatively regarded as stratigraphically underlying the great limestone 

 on the Lone Star ridge. They are unconformably overlain by the Rossland 

 lavas as developed east of Sayward. Between Nine-Mile and Twelve-Mile creeks 

 a strong and persistent band of silicious limestone is intercalated in the phyl- 

 lites; it is truncated at each end by the overlapping lavas in such a way as to 

 illustrate the unconformity. (See map.) 



The structure of this area is fully as complex as that east of the Salmon. 

 The schists are well exposed for miles in the canyon of the Pend D'Oreille, 

 where the dips and strikes of the schistosity planes were seen to shift every few 

 hundred feet. On the average the strike runs a little north of east, so that the 

 river section is not favourable to the discernment of the field relations. 

 Numerous acid and basic intrusions have also affected the structural relations. 



As a negative result of the field work among the schists it may be stated 

 that the leading problems regarding their age, their subdivision into recogniz- 

 able members, and their thickness must apparently be solved outside the ten- 

 mile belt. It is most probable that, if ever found, the key to these secrets will 

 be disclosed on the United States side of the Boundary line. In the present 

 report the whole assemblage of phyllites, quartzites, traps, and limestones is 

 included under the name, Pend D'Oreille group. Its minimum thickness is 

 believed to be 5,500 feet. 



Correlation. — The marbles and schists themselves carry no fossils, so far 

 as known, but a hint as to their age is found in the fact that lithologically 

 similar marbles bearing a Carboniferous species were found by McC'onnell and 

 by the writer in the Rossland district.* In central Idaho, eighty to one hundred 

 miles to the southeast of the Boundary section at the Pend D'Oreille, Lindgren 

 has found closely allied rocks at several, rather widely separated localities, and 

 at most of them some of the rocks bear Carboniferous fossils.f His description 



* Cf . E. G. McConnell. Explanatory notes to Trail sheet issued by the Geological 

 Survey of Canada, 1897. 



t W. Lindgren, 20th. Annual Report, U.S. Geol. Survey, Part 3, pp. 86-90, 1900. 

 25a— vol. ii— 18i 



