ABSTRACTS AND DISCUSSIONS OF PAPERS 59 



cumulations of rock weathering that are commonly involved in slides in moun- 

 tain regions. Their forms are correspondingly varied and complex, in some 

 instances embodying very puzzling mechanical features. The gravity stress 

 which is the fundamental cause of dislocation may be transmitted long dis- 

 tances through the medium of a practically fluid stratum below the zone of 

 rupture, as has not infrequently happened in the Hudson Valley. Unlike the 

 usual condition in mountain forms, there need be no essential variations in 

 the character of the material displaced and the undisturbed beds. Any struc- 

 tural change that could be of significance in the formation of such slides, in 

 the very nature of the case, is scarcely to be looked for, and the same is true 

 also with respect to a slipping surface. 



The conditions attendant on the disturbances can generally be determined by 

 observation or by testing the ground in the vicinity, from which some con- 

 clusion may be drawn as to the causes leading up to the slides. The exact 

 impetus or proximate causes, however, is seldom to be ascertained. Usually 

 several factors have to be taken into consideration in determining the origin 

 of individual slides, and their relative importance is difficult to estimate. The 

 problem may be further complicated by the entrance of some external influence 

 into the situation, either of natural development or arising from the agency 

 of man. 



Of the conditions which govern the form taken by the movement, those of 

 more immediate concern are the nature of the beds — that is, whether clay, 

 sand, or mixture of the two ; the moisture content, and the surface contour. 

 The forms that have come under observation in the Hudson Valley are as 

 follows : 



1. Surface creep, involving soil, sand, and gravel ; little active in plastic 

 clays. 



2. Slumping and flows ; peculiar to clays and silts. 



3. Earth slides; materials of any sort, but not fluent; the movement takes 

 place on the face of slopes that are oversteepened. 



4. Subsidence of surface through squeezing out of a wet clay substratum 

 on the plane of its bed. 



5. Subsidence of surface from unbalanced pressure on confined liquid sub- 

 stratum, leading to an upward movement at a distance. 



The influence of the various kinds of movement on the process of degrada- 

 tion is too important to be left out of account in a region like the Hudson 

 Valley. Their importance, of course, can not be estimated quantitatively, 

 although there is reason to believe that locally they have a predominant part 

 in the work of surface leveling. On the nearly flat tops of the terraces 

 erosion ordinarily is unable to make much headway, especially when the 

 surface is heavily sodded, whereas a very light slope suffices to cause the 

 precipitation of masses of earth in slides, some of which may attain large 

 proportions. There is record of 10 or 12 catastrophic landslides in the Hudson 

 Valley in a period of 75 years ; the larger ones involved upward of 100,000 

 cubic yards of earth. The inconspicuous forms, no doubt, accomplish the 

 largest share of leveling, since they are widely active with cumulative effects. 



Presented in abstract extemporaneously. 

 The Society adjourned at 5.45 o'clock p. m. 



