34 
MUSEUM BULLETIN NO, 25. 
double, a character I have noted also in fossil examples. Some- 
times the whole system of ripples would take the character of 
dunes and in that case the pattern was usually imbricated. One 
might say that the dunes fell into two sets of rows of which the 
directions made an angle less than 90 degrees or that the surface 
was occupied by two systems of ridges, at the intersections of 
which were special developments. 
I also saw something of the process of ripple making, observ- 
ing the eddies in the intervening hollows and the cloud of sand 
projected from each crest as a water wave passed over. 
By the way this reticulated pattern of subaqueous dunes 
is the feature in the Portage formation which Hall called mud 
flow. The material is sand rather than mud and the form is 
developed (at Annisquam) by the joint action of waves and 
current.” 
One of the unusual varieties of ripple-mark pattern is shown 
in Plate XIX B, which was photographed from a mould taken at 
the edge of the beach near Port Colborne on lake Erie. This 
example is the result of the action of very small waves lapping and 
crossing each other from opposite sides of a miniature spit. The 
position of a small partially submerged outward projecting point 
of sand bar supplied the conditions for complex wave action in the 
lee of the bar. 
INTERFERENCE RIPPLE-MARK. 
A form of ripple-mark which has little resemblance to the 
ordinary forms is generally designated as interference ripple- 
mark. It has been called “dimpled current mark” by Jukes. 
It has sometimes been described under the name of “tadpole 
nests” 1 with reference to its supposed origin. In place of the 
parallel trough and ridge structure of the common forms of 
ripple-mark a deeply pitted or cell-like structure is characteristic 
of interference ripple-marks (Plates XXII and XXIII). I have 
found that different examples of interference ripples show a 
rather wide range of characteristics with respect to the appearance 
of the pits or cells comprising them. Just as ordinary ripple- 
1 E. M. Kindle, "An Inquiry into the origin of ‘Batracholdes the Antlquor' of the Lock- 
port dolomite of New York," Geol. Mag., dec. VI, voi. 1, 1914, pp. 158-161. 
