GEOLOGICAL IMPORTANCE OF DUNES. 551 



But wliatever have been the source and movement of these 

 Bands, they can hardly fail to have left on their route some sand- 

 stone monuments to mark their progress, such, for example, as 

 we have seen are formed from the dune sand at the mouth of the 

 Nile; and it is conceivable that the character of the drifting 

 sands themselves, and of the conglomerates and sandstones to 

 whose formation they have contributed, might furnish satisfactory 

 evidence as to their origin, their starting-point, and the course by 

 which they have wandered so far from the sea.* 



If the sand of coast-dunes is, as Staring describes it, composed 

 chiefly of well-rounded quartzose grains, fragments of shells, and 

 other constant ingredients, it would often be recognizable as coast 

 sand, in its agglutinate state of sandstone. The texture of this 

 rock varies from an ahnost imperceptible fineness of grain to 

 great coarseness, and affords good facilities for microscopic ob- 



* Forchhammer, after pointing out the coincidence between the inclined 

 stratification of dunes and the structure of ancient tilted rocks, says: "But 

 I am not able to point out a sandstone formation corresponding to the dunes. 

 Probably most ancient dunes have been destroyed by submersion before the 

 loose sand became cemented to solid stone, but we may suppose that circum- 

 stances have existed somewhere which have preserved the characteristics of 

 this formation." — Leonhard und Bronn, 1841, pp. 8, 9. 



Such formations, however, certainly exist. Laurent tells us that in the 

 Algerian desert there are " sandstone formations" not only " corresponding to 

 the dunes," but actually consolidated within them. "A place caUed El-Mouia- 

 Tadjer presents a repetition of what we saw at El-Baya ; one of the funnels 

 formed in the middle of the dunes contains wells from two metres to two and 

 a half in depth, dug in a sand which pressure, and probably the presence of 

 certain salts, have cemented so as to form true sandstone, soft indeed, but 

 which does not yield except to the pickaxe. These sandstones exhibit an in- 

 clination which seems to be the effect of wind ; for they conform to the direc- 

 tion of the sands which roU down a scarp occasioned by the primitive ob- 

 stacle." — Laurent, Memoire sur le Sahara, etc., p. 12. 



" At New Quay, the dune sands are converted to stone by an oxide of iron 

 held in solution by the water which pervades them. This stone, which is 

 formed, so to speak, under our eye, has been f oimd solid enough to be em- 

 ployed for building." — Esquiros, L' Angleterre, etc., in Bevue des Deux Mondes, 

 1864, pp. 44, 45. 



The dunes near the mouth of the Nile, the lower sands of which have been 

 cemented together by the infiltration of Nile water, would probably show a 

 similar stratification in the sandstone which now forms their base. 



Dana describes a laminated rock often formed by the infiltration of water 

 into the sand-dunes on the Hawaian islands. — OorcUs and Coral Islands, 1873, 

 p. 155. 



