30 LEOPOLD REINECKE 



geological structure. The accordance of topography and structure 

 might in this case be described as a dimmed and accessory char- 

 acter, while the discordance was sharply defined and extensively 

 developed, an essential character. It was found that on an old 

 surface, with slopes of 3 to 6 per cent, the characters developed in 

 maturity were just disappearing and those related to peneplanation 

 were strongly developed but had not yet entirely supplanted the 

 others. 



According to the hypothesis of the geographic cycle this land 

 form, if it had been left undisturbed, would have been gradually 

 worn down; that is, its average slopes would have been gradually 

 diminished, the characters of maturity have gradually disappeared, 

 and the characters of peneplanation have prevailed everywhere. 

 It seemed possible then, by using the criterion of regional slopes, 

 to subdivide old land forms on a quantitative basis. 



The primary object of this paper is to point out the importance 

 of the measurement of average regional slopes upon "old erosion 

 surfaces," and to show that such data assist materially in the more 

 accurate study of the physiographic development of the region in 

 which these surfaces occur, and of the diastrophic movements 

 which have taken place there. The writer believes that it will be 

 possible eventually to subdivide old land surfaces on the basis of 

 their average slopes, and has attempted to do so here. The sub- 

 division proposed is necessarily imperfect, partly because of the 

 lack of accurate data regarding the slopes of old erosion surfaces, 

 and largely because of the writer's imperfect knowledge of the 

 literature describing such surfaces. As more data upon the slopes 

 of old surfaces become available, however, the imperfections of a 

 subdivision of this kind can be remedied. 



METHOD OF MEASURING REGIONAL SLOPES 



By regional slope is meant the general slope of the land toward 

 main drainage lines. Slope is stated here as the percentage of 

 vertical to horizontal distance rather than as an angle, because the 

 measurement of the angles of slope on a land form of moderate relief 

 is generally impracticable in the field, and for that reason the degree 

 of slope stated as an angle, especially if the angle is small, does not 



