142 WAVES OF SAND AND SNOW 



intermediate positions they have an intermediate 

 form, consisting of ridges transverse to the wind 

 with the weather slopes longitudinally furrowed. 

 There are minute shells of foramlniferce, composed 

 of carbonate of lime, in the sand of these dunes. 

 The source of the foraminiferce is the sand blown 

 in from the sea -shore. The percentage of 

 carbonate of lime in the sand decreases with 

 the distance from the sea. 



My theory of the origin of the longitudinal 

 ridges, some miles in length, which occur near 

 the sea is as follows : viz., that when the sand 

 was loose it was laid down in transverse ridges ; 

 that the presence of a large quantity of minute 

 grains of carbonate of lime caused the sand to 

 consolidate or set under the action of moisture ; 

 that the wind then cut through the dunes, forming 

 longitudinal ridges ; and that the sand removed 

 from the surface of the dunes collected from right 

 and left under the lee of these residual ridges and 

 finally united in sequence the longitudinal ridges 

 carved out of successive sand-dunes. The gradual 

 change in habit from' longitudinal to grooved and 

 ultimately to transverse dunes in proceeding inland 

 I attribute to the recorded diminution in the per- 

 centage of carbonate of lime. I intended to have 

 tested this explanation on the spot, but have been 



