RECENT AND FOSSIL RIPPLE-MARK. 
55 
contours for the tops of the ridges as before reversing. It is, 
therefore, evident that current ripple-mark and wave ripple- 
mark with crests and troughs of similar contour cannot be used 
as criteria for determining the true order of superposition in 
areas of complex structure. The type of ripple-mark, which is 
very common in wave ripple-mark, characterized by sharp angular 
crests separated by wide rounded troughs (Figure 7 C, D, and G) 
or rounded hollows, each of which has a low sharp ridge in the 
centre (Figures 4 C and D and 7B) may be depended upon, how- 
ever, to furnish decisive evidence regarding the order of super- 
position in disturbed areas. There appears to be no exception 
to the rule that in the type of ripple-mark in which the profile 
shows a series of gentle curves connected by angles the angles 
will represent the crests. Mr. G. K. Gilbert’s 1 observations on 
this point coincide with my own. Since no examples of ripple- 
marks with rounded crests and angular troughs are known, 
the angular parts of the profile can be considered to represent the 
crests when found in beds whose order of superposition is un- 
known. While the eyidence of this kind of ripple-mark is entirely 
trustworthy it should be clearly understood that the interpretat- 
ion of the evidence of the other types referred to is difficult and 
generally of no value. Prof. Van Hise 2 pointed out some years 
ago the utility of ripple-mark in determining order of super- 
position, but neither he nor Jukes nor Geikie, 3 who discussed this 
principle, indicated that many varieties cannot be used in this 
way. 
Cross Bedding, 
Cross bedding represents in many instances one phase of a 
phenomenon called sand waves which are nothing more than 
current made ripple-mark of mammoth porportions that appear 
to be formed instead of ripple-mark when the current is overloaded 
with sediment. The crests are often 15 to 35 feet apart and 
rise 2 to 3 feet above the troughs. They are formed on the beds 
of most streams which are heavily laden with sediment and 
along most coast lines which are subject to the action of powerful 
1 Bull. Phil. Soc, of Wash,, vol. 2, 1874-78, pp. 61-62. 
* 16th Ann. Rept. Direct U. S. Geol. Survey, Pt. I, 1896, pp. 719-721. 
* Student’s manual of geology, 3d. ed. 1871, p. 63. , 
