PHILOSOPHICAL SOCIETY OP WASHINGTON. 61 



electric light in illuminating the microscope, the structure of the 

 eye and optic nerve, and explaining the theory of such abnormal 

 phenomena, 



Mr. G. K. Gilbert made a communication on 



RIPPLE-MARKS. 



(abstract.) 



Ripple-marks are observed, first, upon dry, shifting sand; 

 second, on sand under water ; third, on sandstone strata. The 

 third case is the fossil phase of the second. 



1. As to their Form. — The ripples on a slab of sandstone are 

 usually equa], parallel, equidistant ridges, curved over the top, 

 and separated by curved troughs ; and the question has been 

 raised whether the curvature of the ridges or that of the troughs 

 is the more acute. Since the stratum which overlies a rippled 

 sandstone may preserve on its under surface a mould, or reversed 

 impression, of the rippling, there may be doubt in the case of a 

 detached specimen whether it exhibits the true ripple or its 

 mould. From an extended series of observations in Utah, the 

 speaker concludes that the crests of the ridges are more acute 

 than the intervening troughs, and that this rule is so little liable 

 to exception, that it may be used in determining which is the 

 originally upper surface of a detached or highly inclined bed of 

 sandstone. An opposite opinion is held by Jukes (Manual, 

 1872, p 163). 



2. As to their Cause. — The view is advanced that the ripples 

 on dry and on wet sand are due to vibrations of air and of water, 

 and are analogous to, if not homologous with, the accumulations 

 of sand along the node lines of vibrating elastic plates. We 

 know from the phenomena of rapids that running water is thrown 

 into vibration by friction on its channel. We know from the 

 whistling of the wind that air is given uniform vibration by 

 friction. Are not such vibrations, arising from the friction of 

 currents, competent, if constant in position, to produce the phe- 

 nomena of ripple-marks ? The following facts appear to accord 

 with this hypothesis : First, the wavelets are, within restricted 

 areas, sub-equal in all dimensions. Second, in the case at least 

 of those formed under water, the wavelet does not travel, like a 

 sand-dune, but is constant in position so long as the conditions 

 remain unchanged. In one observed instance, the lamination of 

 strata showed that a set of ripple-marks had held the same posi- 

 tion while two feet of sediment were accumulated. Third, there 

 are compound ripplings. In one fossil specimen exhibited to 

 the Society, the ripples are double, each main ridge being sup- 

 plemented by a smaller one running along its base. In another 



