OF SAND FOBMATIONS ON MARINE COASTS. 13 



along the lowest lines and leaves ridges and islands, by which its results 

 can be measured. The water currents have, at any one srJot, but a 

 single direction, and the furrows and mouldings of the curved surface 

 are grouped in a single system ; but the wind may blow in many direc- 

 tions, and produces series of corresponding complexity. 



It is a well known law in dynamical geology that all sedimentary 

 deposits are stratified. This lamination is somewhat different when 

 caused by eolian influence than when resulting from the action of water. 

 The sorting power of water is more distinct than that of wind, because 

 of the greater regularity of water currents. 



As a consequence of the rapid variations to which the transporting 

 power of the wind is subjected, eolian deposits are generally straticulate, 

 finer and coarser laminae succeeding each other in indefinite alterations. 

 But there is not the evenness of layer characterizing aqueous deposits, 

 even when made over level surfaces. To make beds without straticula- 

 tion would require winds without these irregularities, — little varying 

 and long continuing, — such as few regions have, except those that have 

 winds of too moderate velocity to carry any but the finest particles. 

 The gusty winds also tend, by their denuding as well as transporting 

 work, to make wavy rather than plane upper surfaces. Moreover, any 

 barrier, as a projecting rock or a stump, or a tuft of grasses, causes 

 a heaping of the sands around the obstacle, and makes curving surfaces 

 in the heaps, owing to the eddies in the air. 



We must here consider the following kinds of lamination : 



1) horizontal 



2) oblique 



3) flow- and plunge 



4) irregular. 



Horizontal strata are developed in water only, especially in non- 

 running water. Each lamination here represents different conditions 

 of the water in which the sediment is deposited, and oblique lamination 

 or cross-bed structure is a result of deposition by rapid shifting currents, 

 carrying material of varying coarseness. While strictly horizontal lami- 

 nation cannot be formed by winds, obliquely laminated layers occur in 

 eolian deposits, indicating that somewhat regular winds have blown 

 for some time. 



This cross-bed structure of the sediments is characterized by a lami- 

 nation in a plane, oblique to the horizon. It results from the pushing 

 along of the sand by currents, causing at first a little elevation, and 



