xviii DEPARTMENT OF TEE INTERIOR 



2 GEORGE V, A. 1912 



CHAPTER XXII. 



Page. 



Physiographic notes on the Forty-ninth Parallel section 599 



Origin of the master valleys 599 



Individual mountain ranges as physiographic provinces 601 



Front range syncline 601 



Galton-MacDonald horst 604 



Question of a Tertiary peneplain in the Rocky Mountain system. 605 



Purcell compound horst 610 



Nelson range monocline 612 



Bonnington-Rossland mountain group 613 



Christina range and Boundary Creek district 614 



'Midway volcanic district 615 



Interior Plateaus 616 



Okanagan range 618 



Hozomeen range 620 



Skagit range 621 



Question of a general Tertiary peneplain in the Cascade mountains. . 621 



Development of accordance of summit in alpine mountains. . .... 631 



Explanations by inheritance 632 



Spontaneous development of summit-level accordance 635 



Summary 641 



General conclusions on the physiographic history of the Cordillera at 



the Forty-ninth Parallel 643 



CHAPTER XXIII. 



First calcareous fossils and the origin of the pre-Silurian limestones. . . . 643 



Introductory; abstract of chapter 643 



Explanations of the unfossiliferous character of the pre-Cambrian sedi- 

 ments 645 



Hypothesis of the metamorphic destruction of fossil remains. . . . 645 



Brooks hypothesis 645 



Suggested hypothesis 646 



Precipitation of lime salts through the decomposition of dead organisms 646 



Duration of the nearly limeless sea 648 



Effects of the Huronian orogenic revolution 649 



Analyses of the Ottawa river 652 



Comparison of the Ottawa and other rivers 654 



Chemical contrast of pre-Cambrian and later river systems 655 



Variations in the calcium supply during and after the pre-Cambrian. . 656 



First calcereous fossils 656 



Tests of the suggested hypothesis. . . . ■ 658 



Corroborative experiments 658 



Observations on the Black Sea 659 



Pre-Cambrian sedimentary deposits 661 



Origin of dolomite and of other magnesian sediments 661 



