)42 THE GEOLOGIST. 



bottom consists. The nature of the material of which these waves 

 are formed is such, that, when the cuiTent ceases, their forms remain, 

 and thus permanently record the direction of the current, which, of 

 coui-se, must have flowed in a line perpendicular to their trend. 

 Thes» wave-like forms are the well-known " ripple-marks," about the 

 origin and nature of which there has often been much misunderstand- 

 ing. They have too frequently been looked upon as having been 

 invariably formed by the action of the waves of the surface of the 

 water stranding on a sandy beach. They are, however, by no means 

 necessarily connected with the waves of the upper surface, but are 

 merely the effect of the movement of the current over the sand, and 

 are the impressions of disturbances of a wave-like form affecting the 

 bottom of the current, and generated by the resistance experienced by 

 it in moving over the sand, under certain conditions of depth and 

 velocity. Stranding waves produce " ripple-marks " on a sandy beach, 

 because they give rise to a current ; and it is this current which 

 produces the ripple-marks. Other facts must be taken into account, 

 if we wish to decide whether any particular ripple-marks were formed 

 by wave-currents, or by currents due to any other cause. 



If we have merely the surfaces of the ripple-marks to guide us, we 

 cannot always determine from which side the current came ; but very 

 commonly their wave-forms move forward and progress in the same 

 direction as the current of water which generates them. This is 

 owing to the sandy material drifted forward by the current being 

 carried up the side of the ripple turned towards the direction from 

 which the current comes, and then thrown down on the opposite side 

 more protected from the action of the current. This will be more 

 clearly understood by means of the diagrammatic section, fig. 2, which 



Fig. 2. 



f e d c b a 



is intended to show the connexion and gradual passage from a level 

 surface at a, to low and round-topped ripples at h, which at c and d 

 become crested and sharp-topped. Except in rare cases such a ripple 

 as r/ could not hold together ; the upper portion would be w^ashed off, 



