1887.] 409 [Shaler. 



has been made the matter of study is that which concerns cycles 

 of deposition. Dr. Newberry and others have given us some very 

 interesting studies on this point. I shall endeavor, however, to 

 show in the sequel that, while these cycles of deposition afford phe- 

 nomena of very great interest and are particularly valuable to 

 those who are endeavoring to unravel the ancient geography of 

 this continent, they constitute 'but a small part of the problem 

 which we have to consider. 



Divisional planes in rocks which have a bedded nature are clearly 

 separable into two categories. First, those which are due to a 

 change in the operation of forces which bring detrital materials to 

 the point where deposition takes place. Secondly, to the opera- 

 tion of forces which in any way serve to make the deposition of 

 this material irregular through the operation of conditions com- 

 ing into action at the point where the deposition takes place. 

 I propose to consider these causes separately, taking first those 

 which are due to an alteration in the circumstances of carriage, 

 by which the sediments were brought to the place where they were 

 built into strata. The most familiar examples of change in this 

 class occur in sandstones : the best illustrations are found in the or- 

 dinary cross-bedded rocks of this nature. In such cases it is easy 

 to see that modifications in the run of currents may be due either 

 to geographic changes of the neighboring shore lines or to altera- 

 tions in the depth of the water in which the deposits have been 

 made. Where the deposits have been brought to their site alto- 

 gether by tidal currents the changes may be due to geographical 

 changes at points very remote from the seat of the deposits in 

 question. Thus in the case of the Mediterranean we have at present 

 processes of deposition going on along the shores in which tidal 

 currents have but slight influence for the reason that the tides in that 

 basin are generally of trifling amount and with few exceptions give 

 rise to but feeble currents. If it should happen that the tides of the 

 Indian Ocean or those of the Atlantic were by geographic change 

 freely admitted to this basin we should have a sudden change in the 

 character of the sedimentary deposits formed along the coast lines 

 it may be a thousand miles away from the point where the change 

 itself occurred. So, too, geographical modifications may profoundly 

 alter the height of the tide in any particular basin. Thus if the 

 barrier which separates the Bay of Fundy from the Gulf of St. Law- 

 rence should be lowered so as to afford a free passage to the 



