REPORT OF THE CHIEF ASTRONOMER 121 



SESSIONAL PAPER No. 25a 



whole thickness locally observed. The highest figure refers to the remarkably 

 extensive outcrop of the Creston rocks at the Yahk river. High as the estimate 

 appears, a minimum thickness of 9,500 feet is assigned to the formation. The 

 estimate is the result of two complete traverses run across the great monocline 

 at this locality. 



It cannot be denied that there may be some duplication in this particular 

 section, but, on the other hand, the writer, after careful study in the field, 

 found not the least hint of duplication. Similarly, in each of a half-dozen 

 other sections in as many different fault-blocks, as much as 5,000 to 7,000 feet 

 of the upturned Creston quartzite were measured without any clue to repetition 

 of the beds. In several fault-blocks the strata stand nearly vertical and errors 

 of mensuration were reduced to a minimum. At McKim cliff, about 3,000 

 feet of nearly horizontal, typical Creston are exposed to one sweep of the eye, 

 with neither the summit or base of the formation to be found at that locality. 



A further indication that the Purcell series, of which the Creston makes 

 up nearly one-half, is enormously thick, is derivable from McEvoy's recon- 

 naissance map of the East Kootenay District.* The map shows that at least 

 3,000 square miles of the Purcell mountain system north of the Boundary is 

 almost continuously underlain by a silicious series evidently equivalent to that 

 cropping out at the Eorty-ninth Parallel. The continuity of the colour repre- 

 senting the series on the map is broken only by patches of gabbroid intrusions 

 doubtless similar to the intrusions so plentifully found in the Boundary belt. 

 When it is remembered that the rocks of the large area in East Kootenay are 

 much faulted and otherwise disturbed so as to present all angles of dip even 

 to verticality, we see certain proof that these conformable strata must have 

 very great total thickness. This conclusion may be corroborated by information 

 won from even the fleeting glance one can give to the rocks that are visible 

 from the railway train on the stretch: from Cranbrook to Kootenay Landing. 

 In minor degree the estimated thickness of the formation may vary according 

 to the somewhat arbitrary position assigned at each exposure to the upper limit 

 of the Creston. In every case th*e formation gradually becomes more ferrugi- 

 nous and thus passes slowly into the overlying Kitchener. The doubtful inter- 

 mediate band of strata often totals several hundred feet in thickness. The 

 top of the Creston has been generally fixed within the band where the thinner 

 bedding as well as the rusty character of the Kitchener becomes pronounced 

 in the quartzitic strata. 



In conclusion, then, the writer believes it to be best to trust the minimum 

 estimate of 9,500 feet for the Creston as embodying the net balance of pro- 

 babilities derived from the field study. It may be add£d that, in the opinion 

 of the writer, this vast thickness for a single formation is not to be explained 

 as only the apparent thickness of beds deposited in fore-set bedding as a sub- 

 marine delta. The recent emphasis of geologists on this source of error in 

 measuring the actual thickness of a clastic formation is certainly justified. 



* Accompanying Part A, Ann. Rep. Geol. Surv., Canada, Vol. 12, 1S99. 



