55 NEW YORK STATE MUSEUM 



thin, even of capillary size, but they afford the only means for the 

 admission of moisture into the lower beds of a thick series of clays. 

 5 Subsidence from unbalanced pressure on confined liquid 

 substratum. The mechanism of the movements in this case are of 

 considerable complexity, and so far as can be ascertained, the only 

 examples of the kind that have been recognized are from the Hud- 

 son valley where they are not uncommon but seem to have escaped 

 general attention. The subsidences are a variation of the preceding 

 type, but as the wet layer lies too deep to find escape on the plane 

 of its bed, it compensates the unbalanced pressure by raising the 

 bottom of the nearest depression or wherever the structure of the 

 overlying beds is weakest. An outflow of liquid clay may occur 

 from under the upraised block as a secondary phenomenon of the 

 disturbance. 



Fig 4 Subsidence arising from subsurface flowage, with compensating 

 uplift of lower ground. A type common in the Hudson valley. 



There is some similarity between the type and that of rock slides 

 produced by artificial excavations which leave an unbalanced load 

 upon an incompetent layer, as illustrated by certain occurrences 

 in the Culebra cut of the Panama canal. These slides, according to 

 D. F. McDonald,^ are characteristic of rocks possessing low crush- 

 ing strength, but high tensile strength. The load produces a down- 

 ward-outward-upward movement somewhat analogous to that in 

 the present case. But fundamentally there is a wide difference in 

 the conditions, as the clay slides involve an upper rigid layer, the 

 compressing medium, and a substratum that behaves practically as 

 a fluid in transmitting the pressure ; the adjustment takes place 



1 Excavation Deformations. Compte-Rendu Cong. Geol. International XII 

 Session, p. 781. 1914. 



