GEOLOGICAL IMPORTANCE OF SEDIMENTATION 529 



perfectly smooth to the teeth. Second, upon a modehng clay giving 

 a fine grit and third, upon clay from New Haven harbor, gray-black 

 when wet, light gray when dry, giving a very small amount of fine 

 grit to the teeth. Lack of time did not permit accurate soil anal- 

 yses to be made of these types. 



The Champlain silty clay, firm and strong when removed from 

 the clay pit, was dried and then covered with water. Within from 

 five to ten minutes a stratum half an inch in thickness would soften, 

 swell, and begin to disintegrate, losing all coherency, turning to a 

 creamy mud and the margins sliding down to the angle of repose. 

 The clay and silty clay were then ground up, mixed in nearly equal 

 proportions, allowed to settle in pans from water and dried in the 

 sun. After becoming thoroughly dry and cracked, two out of three 

 pans were baked over gas, one at a temperature up to 70° C, the 

 other above 100° C, This was done in order to test the effects of 

 drying at and beyond the most extreme temperatures of torrid deserts. 

 Upon being covered with water, the swelling and disintegration took 

 place as before, indicating that a mixture of equal parts of pure clay 

 with silty clay would not preserve the stratum from disintegration, 

 and that the highest ranges of temperature found in nature were 

 equally ineffective. Upon drying, cracking, and then rewetting, 

 the lines of the cracks are partly closed by swelling. The remainder 

 becomes filled and veiled with a more fluid mixture, and upon redry- 

 ing the cracks are established chiefly upon the same lines, indicating 

 a weaker cohesion along the lines of previous cracks. The same 

 feature was observed in the field after a rainstorm, the deeper parts 

 of the cracks having been closed by swelhng, but still forming lines 

 of weakness; the upper parts blurred by the slaking of the mud. 

 Upon wetting in the laboratory, adding a new layer, and redrying, 

 the two layers adhered as a unit, and the cracks were twice as widely 

 spaced. Those which formed, however, followed in general previous 

 lines of cracking, and those cracks which did not reopen were still 

 to a slight extent lines of weakness. Finally, the pure clay m strata 

 half an inch in thickness was dried for a couple of weeks at ordinary 

 temperatures, broken, and placed in water. They gradually soft- 

 ened in the course of an hour, but for days retained their sharp edges 

 and showed no tendency to disintegrate, though swelling 5 per cent. 



