614 DEPARTMENT OF T>HE INTERIOR 



2 GEORGE V., A. 1912 



sequent drainage down the slopes of the orographic blocks of Laramide date 

 must have also been developed. Too little is known as to the bed-rock struc- 

 ture in the region to give certain clues on these questions. Western Sheep 

 creek and the Christina lake valley are apparently located on meridional faults 

 and may represent the erosion channels of consequent streams originally formed 

 on the down-thrown blocks near the fault planes. The western two-thirds of 

 the Coryell batholith is drained by streams in such courses as to suggest that 

 this part of the drainage system is a direct result of the greater ' hardness ' of 

 the batholithic mass as compared with the country-rocks. That is, in this region 

 the drainage once existing on the batholithic cover has been locally replaced 

 by drainage which is centrifugal from the batholith because erosion has lowered 

 the softer rocks all about. Such streams are not consequent on the initial relief 

 of the batholithic cover but are consequent on the intrusion of the batholith, as 

 well as subsequent to the beginning of the erosion cycle affecting the cover. 

 To indicate the composite character of this kind of drainage the writer has 

 proposed the adjective, ' subconsequent.'* The Coryell area does not furnish a 

 very good case of subconsequent streams, in the sense that it is still difficult 

 to prove such origin for them; yet there can be little doubt that the batholithic 

 syenite is harder than the schists and volcanics round about. The course of the 

 Columbia river at the Forty-ninth Parallel is an open problem. It is locally 

 superposed on the Trail granodiorite but almost nothing is known which gives a 

 detailed notion as to the origin of the valley in the batholithic roof. 



Among the many physiographic details of these mountains only one will 

 be here mentioned — the well known system of terraces of the Columbia valley. 

 Simple as these gravel benches are in appearance, their complete history cannot 

 yet be written. Much field-work needs to be done on each side of the Boundary 

 and for hundreds of miles up and down the river, before the facts are sufficiently 

 accumulated. For the present the writer will attempt to do no more than 

 illustrate the most conspicuous terrace of sand and gravel where it occurs at 

 the Boundary line (Plate 73, Figure A). 



Christina Range and Boundary Creek District. — From Christina lake to the 

 Kettle river valley at Midway, the relief and other physiographic features are 

 much like those of the Bossland mountains, and again the systematic portrayal 

 of these features, founded on genesis, has so far proved largely impracticable. 

 The writer has made comparatively little personal study of this region in the 

 field. The facts of relief are already well expressed for an unusual distance on 

 both sides of the Boundary line. The difficult topography of the Boundary 

 Creek District has been contoured with great fidelity by W. H. Boyd of the 

 Canadian Geological Survey, this map serving as the basis for Brock's geologi- 

 cal map of the district.! On the United States side we have the likewise 

 excellent sheets of the Republic and Osoyoos quadrangles of the United States 

 Geological Survey (1904). The topographic materials are, therefore, in hand for 



* Geology of Ascutney Mountain, Vermont, Bull. U.S. Geol. Survey, No. 209, 1903, 

 page 11. 



t Publication No. 828 of the survey, 1905. 



