THE VICTORIAN NATURALIST. 49 



describes similar structures near Fremantle, and ascribes them 

 to " stalactites formed in the sand by the percolation of rain water 

 dissolving and taking up the carbonate of lime found in the sand, 

 and redepositing it in fantastic forms wherever a predisposing 

 cause happened to determine it." This is correct as far as it 

 goes, but it does not go far enough. 



Selwyn, in describing the dunes of the Schanck district, quotes 

 Jukes's remarks with approval both as to character and as to 

 mode of formation. 



Tenison-Woods, in his " Geological Observations," considered 

 that they had nothing to do with trees, while R. Etheridge, jun., 

 described them, but did not discuss their origin, simply calling 

 them concretions. 



Who it was who first divined the true method of their form- 

 ation I am unable to say, but Moseley* has put the whole thing 

 so concisely that a restatement of the case seems scarcely 

 justifiable, excepting in order to give some slight amplification. 

 In speaking of the sand dunes near Cape Town he says :— " The 

 White Sand is calcareous. As it shifts before the wind it, in 

 many places, buries bushes growing near the shore. These die, 

 and their stems, buried in the sand, decay, and in so doing set 

 free a certain amount of acid, which brings about a solution and 

 a redeposit of calcareous matter in the sand. The sand im- 

 mediately surrounding the stems is thus cemented into a solid 

 mass, which encrusts the remains of the bark. The wood decays 

 away and a pipe with a wall of cemented calcareous sand is the 

 result. The sand shifting again, these pipes, which are often 

 branched, are left exposed on the beach." 



The whole subject of the action of these acids is discussed by 

 Julienf in one of those encyclopaedic papers which must always 

 be studied by anyone interested in the weathering of rocks. We 

 shall refer to Moseley's explanation later, and will first consider 

 more closely the nature of the objects and of the rocks in which 

 they occur. 



An examination of a series of specimens will show that in 

 almost every instance they are really tubular structures composed 

 in the mam of carbonate of lime, which forms a ground mass in 

 which are embedded rounded grains of quartz. The proportion 

 of quartz grains will vary in different localities, and will depend 

 on the nature of the original sand of the dune. At times scarcely 

 any quartz will be visible, but the axial part of the tube will be 

 formed of compact, ringing limestone, which is seen to be 

 composed of concentric coats, and which is denser nearer the 

 axis than towards the periphery. Sometimes the structure is 

 solid, the tube being filled up. Externally the tube frequently 



* •' Notes by a Naturalist on the Voyage of H.M. S. Challenger." 



t " Proceedings of the American Association for the Advancement of 



Science," 1879. 



