SOEBY — ON THE STRUCTURES PRODUCED BY CURRENTS. 145 



relative rate must have varied much. In some cases, no permanent 

 accumulation can have taken place ; for simple ripple-waves advanced 

 leaving no bands behind them ; whereas, in other cases, deposition 

 must have gone on at a very considerable rate, for the greater part of 

 their material must have been left behind in the form of thick bands. 

 Sometimes the rate of deposition must have been very uniform, as 

 indicated by the uniform thickness of each band ; whilst still more 

 commonly the rate must have been very variable, for the thickness of 

 each band varies very much in different parts. 



The actual velocity of the current is of course very distinctly 

 indicated by the character of the materials of which either drift- 

 bedded or ripple-drifted layers are composed ; but it is probably also 

 related to other peculiarities in their structure. There are several 

 curious facts still unexplained j but I am much inclined to believe 

 that the velocity of the current has a considerable share in deter- 

 mining the length of the ripples. I have seen cases where the 

 separate ripples were not an inch apart, and others upwards of a yard 

 from each other ; and there must have been some definite cause, 

 more or less intimately connected with the depth and velocity of the 

 current, for this difference. 



Such, then, is a general account of the conclusions to which I have 

 been led by the study of the structures produced by the action of 

 currents. These various structures are so common that they cannot 

 have escaped the attention of anyone who has carefully examined 

 stratified rocks. Nevertheless, it must be admitted that scarcely any- 

 one has studied them as they ought to be studied, or attempted to 

 draw from them the important conclusions to which they lead. A 

 comparison of what may be seen in progress in modern currents of 

 water with the structure of deposits formed at earlier epochs, is suffi- 

 cient to convince anyone that the mere direction of the current can 

 be readily determined in those cases in which its velocity was sufficient 

 to have any decided effect. This alone enables us to ascertain many 

 very important particulars respecting the formation of stratified rocks. 

 It points out the quarter from whence their materials were drifted, 

 and also many of the peculiar features of the physical geography of 

 the period, as I hope to be able to show in a subsequent communica- 

 tion. But this is not all ; for, when strata are deposited under the^ 



