REPORT OF THE CHIEF ASTRONOMER 189 



SESSIONAL PAPER No. 25a 



In brief, it seems to the writer that the facts so far recorded do imply 

 a sedimentary overlap of the Flathead but not a great structural unconformity, 

 or even erosion unconformity, which is general at the base of the sandstone. 

 Considering the size of the area, the observed minute discordances of dip can- 

 not be used safely as positive evidence. The observed failure of beds at certain 

 points can be explained by original wedging-out or by the quite moderate 

 erosion of local upwarps just before the Flathead subsidence. It must be 

 remembered that Middle Cambrian time was exceedingly long. It sufficed for 

 the deposition of 5,000 feet of limestone in the Canadian Rocky mountains 

 at the localities recently studied by Walcott.* During the deposition of such 

 a slowly accumulated sediment, there was evidently plenty of time for local 

 upheavals, considerable erosion, and renewed subsidence along the border of 

 the Cambrian sea. It took only a portion of Pliocene time (next to the Pleis- 

 tocene, probably the shortest of all the major divisions of geological time) to 

 form 5,800 feet of sediments represented in the Merced series of California, 

 a series which itself rests on a Pliocene land-surface.f 



Summary of Conclusions. — In view of the foregoing conclusions the writer 

 does not believe that the pre-Cambrian age of the upper part of the Belt terrane 

 as defined by Walcott, is proved; and regards the Helena-Siyeh formation as 

 probably Middle Cambrian, somewhat older than the Flathead sandstone. On 

 the supposition that the Lewis series and the original Belt terrane have been 

 correctly correlated, that terrane as far down as the upper part of the Greyson 

 shale is tentatively considered to be of Middle and Lower Cambrian age. From 

 the lower part of the Greyson shale to the base of the ISTeihart formation the 

 beds are correlated as pre-Cambrian (pre-Olenellus) but conformable to beds 

 equivalent to the Olenellus zone elsewhere. The name ' Belt terrane ' (or 

 Walcott's 'Beltian'), for the remainder of this report, is restricted to this 

 pre-Cambrian portion of the great geosynclinal prism. 



Table VIII. presents a resume of the writer's tentative correlation of the 

 Forty-ninth Parallel series with the formations described to the south of the 

 Boundary line. 



The Cceur D'Alene series has been tied on to the Purcell and Summit 

 series through lithological resemblances. Calkins has traced the Prichard 

 formation northward, where he found it to pass into the Creston quartzite; 

 we have seen that the Creston is the off-shore equivalent of the Wolf and Monk 

 formations. The special white colour and massive appearance of the Revett 

 quartzite are duplicated in the Ripple quartzite, the two doubtless repre- 

 senting another definite common horizon for the two series. Both series are, 

 in the table, tied on to the Lewis series and thus indirectly to the morp fossili- 

 ferous series on the line of the Canadian Pacific railway. 



The table embodies, with some modifications, the correlation of the Lewis 

 series, Belt series, and. the Gamp Creek-Blackfoot-Ravalli group, as suggested 



* C. D. Walcott, Smithsonian Misc. Collections, Vol. 53, No. 1804, 1908, p. 2. 

 t Cf . A. C. Lawson, Bull. Department of Geology, University of California, Vol. 1, 

 1893, p. 142. 



