RECENT AND FOSSIL RIPPLE -MARK. 
47 
critical velocity of 2*5 feet per second. If cross-bedding is 
found to characterize the beds without ripple-mark it would 
no doubt denote a current much too strong or too irregular to 
produce ripple-mark. 
In the case of the paucity of ripple-mark in the Oriskany 
sandstone still another explanation suggests itself. In many 
places this formation is highly calcareous. In many sections 
where it is at present a pure sandstone the original calcareous 
material has been leached out. It appears most probable that 
the calcareous matter in the sand of this formation would have 
interfered with ripple-mark development during its deposition 
and the absence of ripple-mark in many sections can probably 
be attributed to this calcareous cement. 
It seems from my observations that ripple-mark of large 
amplitude is much more common in limestone than in sandstone 
horizons. Numerous examples have been cited in other papers 1 
of ripple-mark in limestone with an amplitude of one foot or 
more. J. A. Udden 2 has reported ripple-mark in the Lower 
Burlington limestone of Iowa having an amplitude of 4 to 6 
feet and with crests rising 6 inches above the troughs. Sowerby 
mentions 3 to 4 inches as the usual width of ripple-mark crests 
in “medium sandstone” and gives about three-fourths of an inch 
as the smallest and about one foot the greatest amplitude 
observed in sandstone. With one exception, 8 in the Medina 
sandstone, I recall no observation or record of sandstone ripple- 
mark with amplitude exceeding one foot. The sizes commonly 
met with are indicated in Plates XXIV to XXIX. The observat- 
ions recorded in the first part of this paper, however, show that 
the formation of ripple-mark in sand with an amplitude of several 
feet is not uncommon. The extreme rarity of such large ripple- 
mark in sandstone can, therefore, hardly be due to its not having 
1 E, M. Kindle, "Note on a ripple-marked limestone,” Ottawa Naturalist, rol. 26, 1912, 
pp. 1-3, plate VII. 
“A comparison of the Cambrian and Ordovician ripple-mark found at Ottawa, Canada, 
Jour. Geol., rol. 22, 1914, pp. 703-713. 
C. S. Prosser, "Ripple-marks In Ohio limestones,” Jour. Geol., vol. XXIV, 1916, pp. 
456-475. 
1 Geol. Suit., Iowa, Ann. Rept., vol. XI, 1901, p. 87, 
* G. K. Gilbert, "Ripple-marks and cross-bedding," Geol. Soc. Am., Bull., vol. X, 1899, 
pp. 135-140. 
