1888 - 89 .] The Duke of Argyll on Bodies of Organic Origin. 45 
only difficulty is to understand the conditions under which these 
old burrows were so stable as tp he capable of preservation when the 
sandy mud was in course of solidification, and when in many cases 
it must have been placed under the great pressure of later deposits. 
This is one of these difficulties which are felt instinctively even by 
those who do not consciously reason upon it, and it is not surpris- 
ing that it should have long delayed the full acceptance of the 
annelid interpretation of rod-like bodies in a very hard and very 
ancient rock. T do not think that the difficulty of following the 
modus ojperandi in the preservation of these assumed annelid 
burrows has been altogether removed. But undoubtedly some facts 
connected with the peculiar structure and functions of the skins of 
annelids tend to diminish the difficulty in a great degree. W e now 
know that, by virtue of a power to secrete and exude a very glutinous 
slime, some marine worms which do not burrow at all, do neverthe- 
less make for themselves tubes of sand grains agglutinated together 
into a texture so firm that they are capable of very great resisting 
power. The congeners of these worms are in many cases creatures 
which are themselves naked, hut are protected by always burrowing 
under ground. These are said to have, though in a less degree, the 
same power of secreting an agglutinating slime, so that in every 
tube which they dig out, or along which they pass, or in which 
they may lie, they make and leave behind them an internal wall, 
or caked surface, which has a certain coherence — just enough to 
constitute a definite mechanical difference between these tubes and 
the surrounding matrix. This is perfectly intelligible. But it is 
less easy to see how these tubes could come to he filled up by the 
surrounding sand, and yet without obliterating the impression of the 
walls. It seems as if the interior of the tube could only he filled 
by the destruction of its walls — unless indeed, we are to suppose 
that sand of some other texture, not agglutinated by slime, and not 
discoloured by contact with organic matter, had somehow come to 
be introduced into the tube from the top or surface or external 
aperture, and so forced down all its length as to take a cast of it 
along all its course. This really would seem to he the only process 
by which we can explain those cases in which the matrix is coloured 
red, and the tube converted into a rod of pure white quartz rock. 
It is not very easy to follow in detail the natural process by which 
