RECENT AND FOSSIL RIFFLE-MARK. 
51 
Ohio. It has been shown by Prosser, 1 Cushing, 2 and others to be 
separated from the Bedford formation below it by an uncon- 
formity. It may be regarded as representing the lacustrine or 
lagoonal conditions which initiated the Carboniferous period 
in northern Ohio. 
There are other formations in which the evidence of ripple- 
mark, with reference to marine or continental conditions, is not 
so clear. In the Potsdam sandstone of Ontario and New York, 
for example, the ripple-mark evidence does not admit of such easy 
interpretation as in the Berea sandstone. Both symmetrical and 
asymmetrical ripple-mark (Plates XXV A and XXVI I A) are found 
in the sections which I have examined. In the South March section 
west of Ottawa 35 or 40 feet of this sandstone are exposed. In 
the lower 10 feet of the section Lingulepis acuminata Con. and 
Scolithus canadensis are abundant fossils. In the higher beds, 
however, fossils of any kind appear to be entirely absent, while 
ripple-mark which has not been noted in the fossiliferous beds 
appears. Both the symmetric and asymmetric types are present. 
The former seems to be more common than the latter. It is 
perhaps not safe to offer any positive opinion regarding the 
inferences to be drawn from the presence of the two types of 
ripple-mark in closely associated beds. The facts noted appear 
to suggest, however, that most of the Potsdam beds originated 
under non-marine conditions. 
The numerous beds of symmetrical ripple-mark which are 
so common in the Potsdam could not possibly have been produced 
by tidal currents. If these beds were chiefly marine sediments 
and if their sands had been subjected to the action of tidal cur- 
rents asymmetrical ripples should greatly predominate over the 
symmetric type. The symmetric type, however, appears to 
be the dominant form and the presence of the asymmetrical 
ripples seems to be most easily explained as the result of lake 
currents or of tidal currents representing occasional marine 
connexion with lagoonal waters. The entire absence of marine 
fossils from most sections of the Potsdam in Ontario and the 
presence in others of fossil trails which have not been correlated 
1 Geol. Surv,, Ohio, Bull. 15, 1912, pp. 105, 252. 
1 Bull. Geol. Soc. Am., vol. 26, 1915, p. 155. 
