50 THE VICTORIAN NATURALIST. 



passes quite gradually into the loose dune sand. A little search- 

 ing among the fragments of the pipes which lie strewn over the 

 surface of the sand will soon be rewarded by finding some 

 specimens in which there loosely lie in the cavity of the tube one 

 or more dried and decomposing roots of a shrub. Which is first, 

 the tube or the root ? Was the tube formed round the root, or 

 did the root find the tube ready made and pass down it ? This 

 is a question to which we shall return later. 



The next point to consider is the nature of the sand of those 

 dunes in which these structures are found. An examination with 

 a hand lens will show that it is very far indeed from being 

 composed entirely of quartz grains, but that a very large pro- 

 portion consists of small polished fragments of shells and other 

 calcareous organisms, the proportion, it is almost needless to 

 remark, varying with the locality ; thus, in some of our Victorian 

 dune sands there is said to be more than 90 per cent, of lime, 

 while at Sorrento a very rough estimate of a sample with the 

 imperfect means at my disposal gave me somewhere about 50 per 

 cent, Mr. W. H. Green, M.Sc, tells me that a piece of the dune 

 rock from Sorrento which he subsequently examined for me 

 contained 48 per cent, of lime. 



Speaking in a general way, then, and without fixing our atten- 

 tion on the exact percentage of lime in the tubes, we see that 

 they only differ from the loose sand in the fact that the lime they 

 contain is no longer in the form of separate grains, but is in a 

 solid mass. We must look for something which will dissolve 

 carbonate of lime and redeposit it ; and, moreover, as there is 

 such a manifest resemblance in the form of the objects in 

 question to roots, we may point the finger of suspicion at the 

 roots themselves and inquire what they have to do with it. 

 Darwin's explanation was, as is quoted above, that the sand was 

 compacted into rock by the action of percolating calcareous 

 matter, that the roots of the plants decayed and left a cavity 

 which was subsequently filled with lime precipitated from 

 solution. But the pipes occur in loose sand, which would not 

 leave a cavity on the removal of the roots, which is a fatal 

 objection as far as our specimens are concerned. The origin of 

 the percolating calcareous matter is, moreover, left in doubt, and 

 is a question we must discuss. If we consider the products of 

 decomposition set free during the decay of roots, and remember 

 that acids are excreted by roots during life, we shall find 

 that we have an agency at hand which will satisfy all re- 

 quirements. We know that the roots of a living plant contain 

 an acid sap which is capable of acting on substances with which 

 the cells are in contact ; thus a piece of polished marble in a 

 flower-pot will be found to have an imprint of the rootlets etched 

 on its surface. But during decay a still greater supply of acid is 

 set free. Decomposition is in the main a process of oxidation, 



