488 PROCEEDINGS OF THE NEW YORK MEETING 



It will be noted that the Leda clay here contains a considerable proportion of 

 silt as compared with the argillaceous matter proper, bearing out an observation 

 made to the same eft'ect on inspection of the locality. The large amount of silty 

 matter present would render the clay unusually permeable, and it seems, there- 

 fore, to be probable that the water saturated the mass by descending directly 

 through it from the surface, in a manner which would not have been possible in 

 the case of the more purely argillaceous clays of the same age usually found. 



In Rankine's Civil Engineering it is stated that "the presence of moisture in 

 earth to an extent just sufficient to expel air from its crevices seems to increase 

 its coefficient of friction slightly ; but any additional moisture acts like an unguent 

 in diminishing friction and tends to reduce the earth to a semifluid condition, or 

 to the state of mud." It appears probable that in this particular instance the 

 silty clay, surcharged with water, stood in a condition of unstable equilibrium, re- 

 taining its solidity merely by virtue of its unbroken molecular texture, and that 

 at the moment in which it became subject to internal movement this texture gave 

 way and it lapsed into a nearly liquid mass, the particles rearranging themselves 

 with some freedom in the water previously locked up in its pores. 



The fact that many clays when once completely dried and then immersed in 

 water lose their plastic character and crumble down into an incoherent mud, shows 

 that the natural texture is an important element in their coherence and plasticity, 

 and one which does not appear to have been fully recognized in connection with 

 experiments on clays and soils. 



The high specific gravity of the fluent portion of the mass in this case, no doubt 

 enabled it to carry the unbroken blocks of clay along that were supplied by the 

 collapsing sides of the crater-like depression which was immediately formed, and 

 when not subjected to stress these blocks continued to retain their original firm- 

 ness and form. 



The fact that the great mass of moving material was discharged through a com- 

 paratively narrow orifice, shows that the bank of the valley through which it 

 passed was much firmer in character than the clay forming the subsoil of the 

 plain behind. This no doubt arose from the natural drainage of the clay along the 

 bank preventing its complete saturation. The same explanation no doubt accounts 

 for the northern limit of the collapsed area occurring along the line where the sur- 

 face begins to slope down toward the hollow of the old landslip already men- 

 tioned, but the limiting causes on the east and south are not clearly apparent. 



Inquiries made on the spot showed that no excessive rains had occurred inune- 

 diately preceding the slip, but that a great deptli of snow lay upon the ground 

 during the latter part of the preceding winter. These statements are confirmed 

 by the meteorological observations made at Quebec, about 40 miles distant, which 

 have been obligingly furnished by 5Ir R. F. Stupart, dii-ector of the meteorological 

 service. From these it appears that the total precipitation (in rain or melted 

 snow) for the months of November and December, 1897, and in January, March, 

 and April, 1898, was slightl}' below the normal for the past 24 years, but that in 

 February, 1898, it was two inches above tlie average, in the form of an abnormal 

 excess of snowfall in that month of 17.9 inches, the total snowfall for February, 

 being 44.2 inches. The ground was thus heavily burdened with snow in the later 

 winter. During April most of this melted and the soil itself thawed, permitting 



