EH. M. Kindle—Pit and Mound Structure. 545 
photographed after settling two hours. Half of the same mixture 
after being shaken was placed in a vessel 10 X 6 inches, and allowed 
to settle in order to see how the increased lateral extent and decreased 
(4 in.) depth would affect the currents in the body of the mixture 
and the resulting surface features. In this shallow vessel only a few 
small and poorly developed vents appeared. 
SuMMARY AND Discussion. 
The experiments which have been described show that sedimentation 
in salt water, when very fine sediments like mud are involved, is 
accompanied by the development of vertical currents. Parallel 
experiments with fresh water showed no evidence of this phenomenon. 
These upwelling currents keep open vertical tubular channels through 
the subsiding sediment, and develop in some cases around the mouths 
of their upper extremities shallow pits, somewhat resembling in outline 
raindrop impressions, but differig through lacking the sharply 
defined rims which characterize rain-prints (Fig. i) “Under certain 
conditions these vertical currents, instead of ending in shallow basins, 
build up around their upper terminals mound or chimney-like 
protuberances which have the appearance of small concretions or the 
work of burrowing invertebrates. Under natural conditions these 
vertical currents would probably be effective in developing surface 
features on the sediments only where deposition was very rapid, as in 
the delta area of a large river. Where the deposition of mud is going 
on at the rate of 12 inches in four days, as has been reported on parts 
of the Gulf coast, the features which have been described would be 
very likely to be developed. It is quite possible that the structures 
described when developed under marine conditions would be much 
larger than any which can be produced under laboratory conditions. 
Certain curious funnel-shaped depressions which were observed by 
Mr. L. D Burling and myself on a mud-bar in one of the Bay of 
Fundy estuaries, near Truro, N.S., are believed to belong to this class 
of phenomena. The largest ‘of these depressions were 19 inches wide 
and 4 inches deep, while the smallest were about one-fifth of this size. 
At the time they were examined the origin of these very regular 
funnel-shaped pits was a puzzle to both observers. 
The vertical tubular canals which are developed under the 
conditions which have been described would doubtless be closed in 
nearly all cases by the pressure of the adjacent sediment as soon as the 
ascending currents ceased. The small pits or mounds which are 
formed about the upper ends of these channels would, however, 
remain at least for a time, and under favourable conditions might be 
preserved as permanent features on the surfaces of the strata. The 
courses of the vertical canals, although partially or completely 
closed, might remain the routes of maximum movement of waters 
percolating through the sediments for an indefinite period. When 
such waters carried with them and deposited mineral matter, vertical 
concretions cylindrical in section like the remarkable log-like 
1 Louisian Gulf Biological Station Bull. 3, p. 28, 1905. 
DECADE VI.—VOL. III.—NO. XII. 35 
