122 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



In the present case, however, the criteria of inclined fore-set bedding, in con- 

 trast to practically horizontal bedding on a subsiding, flat sea-floor, do 

 Bot seem to be matched by the facts. The prevalence of sun-cracks, ripple- 

 marks and other shallow-water markings in the perfectly conformable Kitchener 

 and Moyie formations, as well as in the Creston formation in less degree, 

 appears to show that the sea bottom and the bedding planes of the sands and 

 muds were nearly level throughout the deposition of the Purcell series. 



The Creston formation is no more extraordinary for immense thickness 

 than it is for its wonderful homogeneity in any one section. There is a signal 

 absence of well-marked lithological horizon-markers. The nearest parallel 

 to this homogeneity among the Boundary formations is that afforded by the 

 ba3al arkose member of the Cretaceous section at the Pasayten river. The lack 

 of strong horizon-markers is not to be explained by the lack of sufficient out- 

 crops; the frequent recurrence of the Creston rocks among the fault-blocks, 

 coupled with the excellence of exposure for portions of the formation in each 

 large outcrop, render it improbable that important bands of rock other than 

 the staple quartzite have been overlooked in the Boundary belt. The forest 

 cap interferes much more with determinations of total thickness and of the 

 larger structural features such as faults and folds, than with the study of the 

 details of composition. Neither in slide rock nor in gravels of the canyon- 

 streams of the areas mapped, as underlain by the Creston, was any other rock 

 discovered in large amount than those which are the dominant components of 

 the Creston quartzite as hereafter described. 



While relative homogeneity characterizes the formation from top to bottom 

 at any one exposure, noteworthy changes in its constitution were observed as the 

 Boundary belt was traversed from west to east. The Creston as outcropping 

 in the Moyie range and western half of the Yahk range thus stands in a certain 

 lithological contrast to the same formation where it crops out farther east. 

 For the understanding of this important fact it is convenient to recognize two 

 different phases of the formation in the Purcell mountain system — a western 

 and an eastern phase. 



Western Phase. — At McKim cliff and in the outcrops immediately east of 

 the crest the material was largely gathered for the following description of 

 the Creston formation in a typical section representing the western phase. 



In. the cliff itself the staple rock is a very hard and tough quartzite, break- 

 ing with a sonorous, almost metallic ring. The individual beds vary from a 

 few inches to twenty-five feet or more in thickness, averaging perhaps three 

 feet. Very often the more massive plates are seamed with thin dark-gray 

 laminae of once-argillaceous quartzite or metargillite, but true shale or slate was 

 never seen in this part of the section. For 2,500 feet measured vertically up 

 the cliff the quartzite, which dips 3°-10° eastward, is specially massive, giving 

 the effect of superb cyclopean masonry, broken horizontally by widely spaced 

 bedding-planes and broken vertically only by master joints. Toward the top 

 of the cliff the rock is somewhat thinner-bedded, but is still a strong, typical 

 quartzite. 



