544 E. M. Kindle—Pit and Mownd Structure. 
of the vessel, another clay mixture in water was prepared, and placed 
in an ordinary table glass with vertical sides. The mixture was left 
to settle after adding a level teaspoonful of salt. After two hours the 
suline mixture showed at the top 12 inches of perfectly clear water, 
while another glass with a duplicate mixture, but without salt, showed 
no indication of clearing. This experiment gave no indication of the 
ascending currents along the sides of the glass seen in the bottle, 
hence the strength of these currents in the bottle must be ascribed 
to the shape of flaring neck expanding downwards. In the glass 
as in the bottle, however, the chimney-lke tubes opening into 
miniature craters and mounds were developed over the surface. But 
instead of being uniformly distributed over the surface as in the 
bottle, they were grouped around the sides of the glass and near the 
centre of the surface of the sediment. The largest one had a diameter 
of #inch. Observation of the pin-hole openings in the centre of 
each of these tiny structures showed ascending currents of water 
which could be recognized by occasional particles of sediment 
coming up through them. After fifteen hours the upper 22 inches of 
the glass was perfectly clear. The other glass was opaque from the 
top down, showing no evidence of the beginning of settling. 
In order to further study the action of the upwelling currents 
which produce the small mounds and pits on the surface of the 
subsiding sediment another mixture of the clay and water to which 
a small quantity of salt had been added was placed in a tube, 
30 X inches. The tube affords the best possible means of observing 
the strong upwelling current action developed in a mass of subsiding 
sediment if it is set in a position inclined a few degrees from the 
vertical. Instead of the numerous miniature currents which would 
develop if left in a vertical position a single strong current then 
develops on the upper side of the tube, which carries upward a steady 
stream of the small particles of sediment. The current is apparently 
the resultant of the gradual downward movement of the whole 
subsiding mass of sediment in the upper part of the tube. This mass 
had subsided 14 inches in about 13 hours. Intwo days it had subsided 
204 inches. On the third and fourth days subsidence amounted 
to only 1 inch, and then dropped to 4 in. per day for two or 
three days before becoming imperceptible. These currents belong 
apparently to the class of phenomena called convection currents,’ 
although there is no noticeable temperature difference between the 
upper and lower parts of the mixtures in which they are produced. 
In some cases the current builds a mound-shaped mass about the 
mouth of the tube ; in others a circular pit with a rim about the 
margin develops instead of the mound. Sometimes, instead of either 
of these, pin-head depressions appear over the surface. Beehive- 
shaped protuberances with a small opening at the top are the most 
common. The appearance of the peculiar surface which is developed 
on a precipitate in which salt is the accelerating factor in throwing down 
the sediment is shown in Figs. 14 and 8. The clay mixture shown in 
this Figure was placed in a vessel 63 inches deep, which it filled and 
1 Robert B. Sasman, ‘‘ Types of Prismatic Structure in Igneous Rocks ”’ : 
Journ. Geol., vol. xxiv, p. 219, 1916. 
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