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MUSEUM BULLETIN NO. 25. 
from wave action, and still another type from the direct action 
of the wind on sand. If these different types can be recognized 
by the geologist in examples of fossil ripple-mark, their dis- 
crimination will in many cases afford important aid in deter- 
mining the history of the formations with which he has to deal. 
Marine deposits of the coastal zone are subject almost con- 
tinuously during their formation to the action of tidal currents; 
while in the formation of lake deposits the influence of such 
currents is absent and the presence of other currents is com- 
paratively rare. The preponderance, therefore, in a given 
formation of the wave-made type of ripple-mark, would furnish 
strong evidence of continental origin. The general occurrence 
on the other hand of current made ripple-mark and absence of 
the wave type would afford very strong evidence of marine 
origin. 
The present paper aims to present the criteria by which 
the many varieties of ripple-mark met with in nature may be 
referred to a few general types. These distinctive characteristics 
are shown chiefly through the use of photographs and profiles 
from casts of plaster of paris moulds taken directly from ripple- 
mark shortly after its formation and from fossil ripple-mark. 
The conditions under which the recent ripple-mark thus 
illustrated has been formed have in most cases been carefully 
observed. Many of the moulds have been taken under water by 
means of specially devised apparatus. 
A distinct relationship exists between the physical geog- 
raphy of modern seas, lakes, and estuaries and the currents and 
waves within them. Currents and waves so impress themselves 
on the deposits formed under their influence that their char- 
acters may be recognized from the peculiarities of these deposits. 
Hence through the correct interpretation of the physical features 
of ancient sediments in terms of the current and wave factors 
which moulded them, the features of the attendant physical 
geography can be inferred within certain limits. 
Geological literature abounds with references to ripple- 
mark which indicate a lack of knowledge of the different types 
of ripple-mark and of the varying conditions under which they 
are produced. In a paper published recently the surprising 
