542 E. M. Kindle—Pit and Mound Structure. 
IiL.—Smatt Pir ann Mounp SrrucrurEs DEVELOPED DURING SEDI- 
MENTATION! 
By EK. M. KINDLE. 
(PLATE XIII.) 
IntrRopUcTION. 
EDIMENTARY rocks show a variety of minor structures whose 
k-) origin has been obscure. Among these are the small circular 
pits and mounds which sometimes mark the surfaces of sedimentary 
strata. Such of these as cannot be referred to rain-prints or the work 
of worms are apt to be referred by the geologist to some obscure type 
of concretion. 
Since the publication of Lyell’s? excellent description of recent 
and fossil rain-prints most of the small shallow pits and mound- 
shaped protuberances met with on the surface of stratified rocks have 
been ascribed to the work of raindrops. Lyell in describing rain- 
priits mentions also certain small protuberances resembling rain- 
print casts on dried ripples of mud from the Bay of Fundy, which 
Faraday duplicated Gaia iene by introducing air into the lower 
part of a tube of mud. These on bursting gave cavities resembling 
in size the ordinary rain-prints, but in nearly all cases without any 
rim projecting above the general surface as in rain-prints. | 
In the present paper it is proposed to describe processes of sedi- 
mentation observed under laboratory conditions which result in the 
formation of miniature pits or mounds at the top of chimney-like 
tubes in the falling sediment. Some of these are comparable with 
raindrop impressions and may have been frequently mistaken for 
rain-prints when met with in consolidated rocks. It is believed too 
that the processes observed in the experiments to be described should 
be given consideration as possible factors in the explanation of certain 
types of concretions. 
The experiments on which this paper is based illustrate an 
important serie: of phenomena attendant upon sedimentation which 
are allied to. if not identical with, those known to chemists under the 
general term catalysis. These include changes in water, suspended 
sediment resulting in flocculation and deposition, which are brought 
about by the presence of agents which themselves remain stable and 
are not precipitated. In other words, the active or predisposing 
agent does not unite with the precipitate. Professor Brewer,? who 
was one of the first to investigate this subject, found that very fine 
sediment may remain in suspension for as much as six years when 
left undisturbed in fresh water, while the same sediment might be 
precipitated in a few minutes in sea-water. With some clays the 
precipitation is as much in thirty minutes in salt water as it is in as 
many days or months with fresh water. A considerable amount of 
1 Published with the permission of the Director of the Geological Survey of 
Canada. 
2 “On Fossil Rain Marks of the recent Triassic and Carboniferous Periods”? : 
Quart. Journ. Geol. Soc. vol. vii, pp. 238-47, figs. 1-8, 1851. : 
3 Wm. H. Brewer, ‘‘ On the Subsidence of Particles in Liquids’’ : Memoirs 
Nat. Acad. Sci., vol. ii, pp. 163-75. 
