50> NEW YORK STATE MUSEUM 



Upon the jointed blocks. Small slides thus produced have been 

 observed in actual progress by the writer on the face of a new- 

 formed bank, where the beds had been left in a jointed condition, 

 Mather^ attributes the formation of the slide at Troy on January 

 I, 1837, to the opening of a fissure at the top of the bank into which 

 surface waters found access, thus producing a pressure at the back 

 of the mass sufficient to dislodge it. 



4 Subsidence from squeezing out of wet substratum. The 

 distinctive feature of this type of disturbance as compared with 

 those already described is in the combination of a vertical move- 

 ment or subsidence at the surface and a lateral flowage in the 

 underlying substratum. The latter, rendered mobile by saturation 



Fig. 2 Earth slide caused by soft layers between firmer beds 



with water, finds escape along the plane of its bed by extrusion on 

 the face of a slope or bank where it outcrops. On the other hand, 

 if the saturated layer lies below the lowest point of the neighboring 

 depression, the conditions are favorable for the occurrence of 

 slides of the next class. 



In subsidences of this kind the surface materials have to be 

 sharply differentiated from the underlying clay stratum in their 

 capacity for flowage. The former behave as a relatively firm 

 coherent layer which by its weight exerts a compressive strain 

 upon the mobile mass, squeezing this out in a horizontal direction. 

 As the clay escapes by extrusion, the upper layer sinks down along 

 one or more fracture lines just as in the faulting of rocks. The 



1 Geology of New York. Ft i, The First Geological District, p. 33^34- 



