38 
MUSEUM BULLETIN NO. 2. 
At Black Rock, N.S., where these observations were made, 
the mud-cracks when made in the lower parts of the mud flats 
are obliterated by each tide owing to the heavy deposit of 
sediment left. In a higher zone which is under water for a 
shorter period, the mud-cracks are only partially masked by 
the sediment left by a falling tide. In a third and highest 
zone, mud-cracks may persist through several tides because of the 
shorter period of submergence and lighter deposit of sediment. 
The accompanying photograph of mud-cracks (Plate III, fig. 2) 
taken at Black Rock, is introduced here to show that mud- 
cracks formed on the higher portions of the littoral zone are not 
evanescent features requiring very special conditions for preser- 
vation as they are often assumed to be. The mud-cracks here 
shown lie between 3 and 8 feet below high tide and had 
been covered when photographed, by at least two tides. They 
had also been exposed to a heavy shower of rain immediately 
before the photograph was taken, and are cut across by resulting 
rills as shown in the photograph. Yet notwithstanding the 
deposit of sediment from two tides, the action of a strong current 
during ebb and flow, and the beating of rain, they remain dis- 
tinctly outlined instead of being obliterated as might have been 
expected. A few days of cloudy weather would, of course, result 
in their obliteration by fresh sediment. There is no evident 
reason, however, why these mud-cracks which occupy the outer 
zone of tidal action might not, if they occurred in an arid climate, 
continue indefinitely to receive their daily deposits of sediment 
from the rising tide. So long as the mud-cracks were not 
obliterated by new sediment the recurrent daily shrinkage 
resulting from the exposure to the sun would be more likely to 
keep open the old cracks than to open new ones. The semi- 
permanent character which is assumed for the mud-cracks in an 
arid climate would produce the polygonal cracking of the beds 
which would eventually result in columnar structure being 
impressed upon each tidal deposit as it was laid down. 
It is believed that the columnar structure of the limestone at 
Mount Wissick originated in this way. The shell of sediment 
surrounding each column strongly supports the inference that 
the columns are the result of mud-cracks which extended to a 
