CAMBRIAN AND ORDOVICIAN RIPPLE-MARKS 'jog 



built beds consists largely of small fragments of crinoid stems and 

 other fossils. 



Comparison of limestone and sandstone ripple-marks. — -These 

 Trenton limestone ripple-marks show a striking contrast with those 

 of the Cambrian sandstone which have been described, in the matter 

 of size. Instead of an amplitude of 2 or 3 inches which characterizes 

 the Cambrian ripple-marks, these have a width of 2 or 3 feet from 

 crest to crest. This discrepancy in size is of interest because it exists 

 between the great majority of the examples of limestone ripple- 

 marks and sandstone ripple-marks which have come under my 

 notice. I have observed very few cases where the amplitude of 

 ripple-marks in sandstone exceeded a few inches.^ On the other 

 hand, six of the eight photographs in my collection illustrating 

 limestone ripple-marks, which represent as many different geo- 

 logical horizons and widely separated localities, show an amplitude 

 of more than one foot. The data given in geological literature rela- 

 tive to limestone ripple-marks are often too incomplete to furnish 

 data on the amplitude, but where given the amplitude is likely to 

 be represented by a figure much larger than those which usually 

 represent the amplitude of sandstone ripple-marks. 



The earliest description of limestone ripple-marks with which 

 I am acquainted relates to ripple-marks of large amplitude. This 

 occurs in Dr. John Lock's description^ of the "Blue Limestone" 

 (Ordovician) of southwestern Ohio. Under the head of "Waved 

 Strata," he described a stratum in which "the upper side is fluted 

 out in long troughs 2-3 feet wide and about 2 or 3 inches deep, 



the edge or ridge between them being generally sharp These 



waves are not local, but may be traced in the same stratum over 

 tracts of many miles. "^ 



Dr. August Foerste states'* that "wave-marks (ripple-marks) 

 occur in Ohio, Indiana, and Kentucky, in abundance in the Lower 

 Eden, Upper Richmond, and upper Brassfield hmestones. They 



' Mr. G. K. Gilbert has recorded one exceptional case in the Medina sandstone 

 in which the amplitude is reported to be several feet (Bull. Geol. Soc. Am., X, 135). 



^ Geol. Surv. of Ohio (1838), p. 246, PL 6. 



3 Lock expressed the belief that these were not ripple-marks because "all geolo- 

 gists will agree that the blue limestone has been formed far below the reach of ripples." 



1 Letter to the writer. 



