390 DAVIS — PLAINS OF MARINE AND SUBAERIAL DENUDATION. 



G. M. Dawson describes an ancient i)eneplain, now an elevated and 

 dissected plateau, in the Rocky Mountain region ot* Canada : 



" Climbing to the level of this old plateau, or to that of some slightly more ele- 

 vated point about the fiftieth or fifty-first parallel of latitude, the deep valleys of 

 modern rivers with otlier low tracts are lost sight of, and the eye appears to range 

 across an unbroken or but slightly diversified plain, which, on a clear day, may be 

 observed to be bounded to the nortlieast, southwest and south by mountain ranges 

 with rugged forms, and above which in a few places isolated higher points rise, 

 either as outstanding monuments of the denudation by which the plateau was pro- 

 duced, or as accumulations due to volcanic action of the Miocene or middle Tertiary 

 period." '* 



After explicitly considering the alternatives of marine and subaerial 

 erosion, the author decides against the former, because the ])lateau dis- 

 trict is not accessible to the sea, and because there are no marine strata 

 thereabouts referable to the period when the peneplain was formed. 

 The river system of the region^ — 



" aided by other subaerial agencies, cut down almost its entire drainage basin 

 till this became a nearly uniform ])lain, with some slight slope in the main direc- 

 tion of the river's fiow, but of which the lowest part approximately coincided with 

 the sea-level of the time. . . . After reaching this baselevel of erosion the 

 rivers would, of course, be unable to do more than serve as channels for the con- 

 veyance of material brought into them from the surrounding country, which, 

 wherever it stood above tlie general level, was still subject to waste. The val- 

 leys became wide and shallow, and the surface as a whole assumed permanent 

 characters." t 



My own studies lead meto believe thatsubaerial denudation has reduced 

 various mountainous or [)lateau-like uplifts to lowland peneplains. :|; 



A considerable number of extracts might be i)resented from the works 

 of foreiiin writers to show that the idea of marine deiuidation is on the 

 whole less favorably received by continental than l)y English geologists ; 

 but the features of land form and the processes of land sculpture have 

 not l)een studied in Europe with the attention that has been given to 

 stratigraphic succession or to the problems of paleontology and [)etrog- 

 raphy. Regions that are known to be uplands of denudation are often 

 described with abundant detail as to their structure, but with the scantiest 

 reference to the conditions of their topographic development. 



* . . . Tlie Rocky Mouiiuiia region in Canuda . . . , Tnuis. Roy. Sou. Can., viii, IS'JU, p. 11 



t Loc. cit., p. l;i. 



|The following articles may be referred to : Relation of the coal of Jlontana to tlie older rocks 

 (Tentli Census U. S , vol. xv, 1S8G, p. 710); Topographic development ... of the Connecticut 

 valley (Am. Jour. Sci., vol. xxxvii, 18S9, p. 43U) ; Geograpiiic development of northern IS'ew Jersey 

 (with J. W. Wood. Proc. Boston Soc. Nat. Hist., vol. xxiv, 1889, p. :i73) ; Rivers of northern New Jer- 

 sey (Nat. Geog. Mag., vol. ii, 1891, p. 6); Topographic forms of tlie Atlantic slope (Bull. (jcol. Soc. 

 Am., vol. 2, 1S9I, p. 5.i7) ; Physical geography of southern New England (Nat. Geog. Monogr., vol. 

 i, 1895, p. 27(3) ; Development of certain English rivers (Loiidon Geog. Jour., vol. v, 18j5, p. Hu). 



