1869.] 239 [Morse. 



space between these ridges is filled with semi-fluid clay, quite different 

 in condition from the same beds exposed in the banks. These ridges 

 indicate that the slide was not simultaneous, but in detached jjortions. 



A portion of the bank nearest the river first falls, the harder clay 

 above, with the sand and compacted soil, holding together. It slides 

 on an incline, the lower portion being crowded into the riverbed; 

 this turns the original ground surface of the mass from the river, and 

 toward the embankment. Another portion falls, forcing up the semi- 

 fluid clay beneath, and perhaps partially burying the first fall, and 

 thus section after section of the land falls until the accumulated debris 

 checks all further progress. As these separate masses fall they force 

 into the river those portions which first fell. In the Ammoncongin 

 slide, the clay and soil were urged down the river quite two thou- 

 sand feet, and up the river nearly sixteen hundred feet. The em- 

 bankments of this slide at its outlet are nearly perpendicular, and 

 over thirty feet in height. At its upper end the embankment is twenty- 

 five feet in height, and rises one foot in five from the surface of the 

 sunken area to the upper surface of the clay strata ; here the embank- 

 ment becomes nearly vertical, the vertical portion being confined to 

 the overlying sand, which is about ten feet thick, and this latter feature 

 obtains around the entire embankment. I am indebted to Mr. 

 Hiram F. Mills, Hydraulic Engineer, for these figures, and for the 

 privilege of reducing the plan of the Ammoncongin slide from his 

 surveys. I have roughly estimated the superficial area of the ejected 

 clay at fifteen hundred thousand square feet. Immediately at the 

 mouth of the slide, and in the centre of the old river bed, the clay 

 stands twenty-five feet above the water level, besides covering a 

 large extent of intervale to a considerable depth. 



Thus while the phenomena have the appearances of a slide, the 

 evidences ai'e that it is only a " slump" or fall, caused by the softer clay 

 beneath yielding to the pressure above, and being forced out by the 

 weight of the superincumbent mass. 



Prof Dana, in his JNIanual of Geology, states precisely the charac- 

 ter of these movements in the following words: "A clayey layer, 

 overlaid by other horizontal strata, sometimes becomes so softened by 

 water from springs or rains, that the superincumbent mass by its 

 weight alone presses it out laterally, provided its escape is possible, 

 and sinking down, takes its place." p. 649. And he cites a subsidence 

 of this kind that occurred near Tivoli on the Hudson River in 1862. 



In a discussion before this Society, participated in by Mr. T. T.Bouve 



