NOMENCLATURE 727 



earlier explorers. For such of these as are concerned in the study of the 

 geology, the following table is prepared. The differences between the 

 formations stated by Walcott as occurring in a mountain and the forma- 

 tions found there are not due to varying interpretations of the same 

 strata, but to the fact that Mr. Walcott did not allow^ for possible fault- 

 ing between adjacent mountains in making visual correlations. (Mounts 

 Titkana and Rearguard, for example), or did not allow enough where 

 the faulting is evident (Mounts Hitka and Mumm, for example) 

 (Table 3). 



Contact betw^een the Iipper Cambrian-^ and post-Cambrian 

 (Ordovictan or Devonian) (§6) 



(§ Ga.) According to Walcott (page 336 of his paper), ^'there is no 

 known well-defined lithological break between the Ordovician and the 

 Cambrian" in the Mount liobson region. He draws the lino tentatively 

 in the isolated series of thin-bedded and shaly limestones exposed in the 

 Extinguisher ("Billings Butte'"), "where there is a commingling of the 

 Lower Ordovician and the Upper Cam1)rian faunas.'' 



This fauna has since been discovered in place in the summit of Mount 

 Rearguard (§§ 18 and 23/), and evidences of disconformity have been 

 observed at the line where the Cambro-Ordovician boundary is now 

 drawn. The break in the section does not appear to be so important as 

 that between the Middle and Upj)er Cambrian (§ T), but it should be 

 mentioned that the Ordovician, as here used, appears to include the 

 Ozarkian of Ulrich, and that we have in our fossil collections many fossil 

 horizons from the beds immediately above and below the boundary as 

 ^ye have drawn it. 



(§ 6b.) The contact between the Cambrian and the Ordovician seemed 

 so important, and the new light obtained on the problem from widely 

 separated districts of the Cordilleran region by the writer Avas so exten- 

 sive, that when the first draft of this paper was written he' expected to 

 postpone that general subject to a sej^arate publication. The collections 

 are not now available to him in his peregrinations, but it will be of inter- 

 est to students to know that the boundary between the Upper Cambrian 

 and the overlying rocks in botli the Canadian Pacific Railway section and 

 the Grand Trunk Raihvay section exhibits certain remarkable features. 



(§ 6c.) As was announced in 1916,^ there is a large area in which the 

 relations of the Upjoer Cambrian and early Ordovician to the Devonian 

 are those of angular conformity, and the detection of the line is so diffi- 

 cult that geologists have united into one group the rocks above and below. 



'^The discussion includes a consideration of the Ozarkian. 

 "Summary Bept. Geol. Surv. Canada for 1015, p. OS, 101 (>. 



