SOIJBY— ON THE STRUCTURES PRODUCED BY CURRENTS. 145 
relative rate must liavo 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 iniiform 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. Thei-e are several 
ciirious facts still unexplained ; but I am much inclined to believe 
that the velocity of the current has a consrderable 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 otjier ; 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 sufii- 
cient to convince anyone that the mere direction of the current can 
be readily determined in those cases in which its velocity was suflicient 
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 
