FIFTH REPORT OF THE DIRECTOR I908 



157 



feet long and from 35 to 50 feet wide was fractured along two 

 approximately parallel lines and the included mass dropped a dis- 

 tance of fully 40 feet along the northern fracture plane. This 

 plane had a slope of 8o° toward the ravine. It showed a smooth 

 surface against which the sunken block abutted so closely as to 

 leave no opening. The walls of the second fracture within the 

 ravine inclined away from each other (due to the decided tilt of 

 the fallen block toward the north and the upraising of the ad- 

 jacent beds) and were separated by a gap 15 feet wide and nearly 

 as deep. 



Vertical section across the fault planes. Original slope of hill is indicated by the dotted 

 line, while the heavy broken horizontal line shows the appro ximite watsr le/3l. 



The clay beds behaved very much like solidified rocks. In their 

 mass they were but little cracked or disturbed by the subsidence. 

 The beds above water level had become fairly dry by evaporation 

 and absorption of moisture by plant growth after the spring rains, 

 so that at the time of the occurrence they must have been firm 

 and tenacious. 



The principal factor in determining the nature of the slip was, 

 no doubt, the presence of an underlying mass of water-soaked 

 semiliquid clay beneath the dry upper beds, approximately fol- 

 lowing the level of the stream. Evidence of this liquid clay was 

 observed in the lowest part of the ravine where a considerable 

 quantity had accumulated by extrusion during the subsidence. 

 The upper beds were thus probably in a state of delicate equilib- 

 rium, ready to be precipitated from any slight cause which seems 

 to have been furnished by opening of the parallel fissures some time 



