572 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



leaves no sedimentary record on or near the Forty-ninth Parallel except, it 

 may be, the Kennedy gravels described by Willis as a snbaerial apron of gravel 

 flung out on the plains from the eastern foot of the Lewis range. At the 

 Forty-ninth Parallel there are no known criteria whereby it can be shown that 

 this part of the Cordillera suffered massive warping such as that supposed to 

 have affected the Cascades in Central Washington during the Pliocene. 



20. One of the last important changes in the constitution of the region 

 adjoining the Boundary belt is the growth of Mt. Baker, a volcano perhaps 

 begun in the Pliocene and continuing its activity through the Pleistocene and 

 Recent periods. 



21. Finally, it should be remarked that the later history of the Cordillera 

 is to be read largely and, for great areas almost solely, as a result of detailed, 

 physiographic studies. These should be as far as possible quantitative and 

 must cover a much wider zone of the Cordillera than that surveyed along the 

 International Boundary. Therefore, in the latest, as in the earliest, and all 

 the middle chapters of the history here outlined, much remains to be done 

 before this part of the mountain-chain is genetically understood. 



Observations bearing on the Theory of Mountain Building. 



The conditions affecting the origin of mountain ranges have long been 

 recognized as offering some of the toughest problems in geology. The diffi- 

 culties of the old contraction theory seem to be enhanced by the discovery 

 that a notable part of the earth's surface heat is due to radioactivity. Assump- 

 tions as to the earth's original temperatures, either in the thin surface shell or 

 crust or in the vast interior, can not now be made with the relative degree of 

 confidence felt by some writers before the radioactivity of nearly all accessible 

 rock-matter was demonstrated. Further complications have recently been 

 introduced by the launching of the planetesimal hypothesis, which has not been 

 developed sufficiently, so that the earth-shell bearing the maximum heat of com- 

 pression can be located or the thermal state of the earth during geological 

 time described. A stable orogenic theory founded on thermal contraction in- 

 volves also a suitable cosmogonic theory of the earth. It is safe to conclude that 

 it will be long before there can be unanimous opinion on the validity of the 

 contraction hypothesis of mountain-building. Other explanations are but frag- 

 mentary and they likewise suffer from our lack of information as to the exact 

 thermal condition of the earth's interior. 



The study of the Forty-ninth Parallel section has led to few novelties in 

 theoretical suggestions on this subject but it has found many new illustrations 

 of recognized orogenic principles. Among the significant facts are the 

 following. — 



1. At the Forty-ninth Parallel, as generally throughout the North Ameri- 

 can Cordillera, each period of orogenic folding has been preceded by heavy 

 sedimentation. Each of the principal folded tracts is located within a geosyn- 

 clinal belt. 



