43 
t 
No. 
Explanation of Plate III. 
Fig. 1. Block of columnar limestone adjacent to the parent ledge. The columnar 
structure extends down about 20 inches in this block. Photograph by 
Olaf O. Nylander, 
Fig. 2. Mud-cracks at Black Rock, N.S., which have been covered by two or more 
tides. Photograph by L. D. Burling. 
