RECENT AND FOSSIL RIPPLE-MARE. 
39 
the sand at its bottom gently along at whatever depth that bot- 
tom may be from the surface of the water.” 1 This entirely 
correct statement removes ripple-mark from the list of features 
which can be regarded as evidence of shallow water conditions. 
Among the earliest observations on the depth to which 
wave action extends are those of Siau. 2 A translation of his 
description of the ingenious methods of observation used and his 
conclusions follow: 
"The observations which we record were made on white 
madreporic and black basaltic sand bottoms. They were made 
during the course of examinations in connection with the estab- 
lishment of a harbour at Sant Gilles where there is a natural 
channel current through the coral reef that lies along the coast. 
"When the sea is calm enough to allow the gravelly sand of 
the channel bed to be seen, parallel ripple-marks may be dis- 
cerned, the transverse section of which changes with the state 
of the sea which causes their formation. On the occasions when 
observation was possible the distance between two consecutive 
troughs or two crests was estimated to be 30 to 50 centimetres, 
the depth of the trough below the crest being about 10 to 15 
centimetres. 
"The heavier materials such as coarse sand, gravel and small 
stones are found in the hollows of the undulations and the finer 
sand at the summits. 
"When the ripples are made up of materials of the same size 
but of differing specific gravities, like basaltic and calcareous 
sand, the heavier materials are found in the hollows and the 
lighter at the summits. 
"It may be readily seen that the undulations are due to the 
actions of the waves. When the water is undisturbed and the 
bottom is visible, the waves have very slight effect on it; but 
when the water is agitated, the materials are set in motion. 
Little by little as the waves decrease, their effective action 
diminishes, until they are no longer able to move the heavier 
i Manual of geology, 1357, p. 172. 
*Siau, “De Taction dea vagnes & de grandes profondeurs," Comptea Rendua Hebdo- 
mad alrea des Stances de TAcad&mie des Sciences. Tome XII, 1841, pp, 770-776, 
