RECENT AND FOSSIL RIPPLE-MARK. 
45 
In the Coal Measures, sections at Joggins and at Sidney, 
Nova Scotia, afford notable examples of ripple-mark horizons. 
While ripple-mark is a very common characteristic of the 
sandstone horizons mentioned as well as of many others, it is 
worthy of note that it is absent or extremely rare in various 
other sandstone horizons. 
Although familiar with numerous sections of the Oriskany 
sandstone along its entire line of outcrop from Ontario to Alabama 
I have never observed ripple-mark in this formation. Prof. 
Van Ingen informs me that he has noted ripple-mark in the New 
York Oriskany, but evidently this phenomenon is rare in the 
. Oriskany as compared with many formations. In the Knobstone 
or Riverdale sandstone of Indiana and Kentucky ripple-mark 
is equally rare throughout its extended outcrop area. The 
Kaskaskia sandstone of southern Indiana is also entirely free 
from ripple-mark so far as I have observed. These sandstone 
formations without ripple-mark are composed of materials which 
closely approach the highly ripple-marked beds of other for- 
mations in general texture and composition. Scores of other 
examples might be given of formations in which ripple-mark is 
either a very rare or a very common feature. The contrast in 
this particular between different sandstone formations raises the 
interesting question: why should ripple-mark be a conspicuous 
feature throughout some sandstone formations and generally 
absent in others ? An explanation of the comparative abundance 
of ripple-mark in some horizons and its scarcity or absence 
from others would doubtless go far toward a complete inter- 
pretation of the complicated history of the beds discussed. 
While an adequate explanation may not yet be practicable, a 
brief consideration of some of the possible reasons for this differ- 
ence may afford valuable clues to some of the factors in the history 
of particular formations. 
In seeking an explanation of the apparent anomaly of the 
great abundance of ripple-mark in some sandstone formations 
and its absence or rarity in others, it is well to remember that the 
preservation of ripple-mark in marine sands is probably always 
the result of a special combination of conditions. If the tidal 
currents which produce asymmetric ripple-mark always flowed 
