CALAMODENDRON. 15 



took place.^ No doubt such nodules contain a fair sample of the plants of which the seams 

 of coal in which they are found were formed ; and their calcification was most probably- 

 due to the abundance of shells afterwards accumulated in the soft mud and then decom- 

 posed, and now forming the shale overlying the coal. 



At present little is known of the process by which animal and vegetable bodies are 

 decomposed, and the particles of which they were formed removed and exactly replaced by 

 mineral matter. All observers have been struck with the wonderful perfection of the 

 process by which the most microscopic parts of minute vessels and cells have been pre- 

 served in form ; but no author could satisfactorily account for it until the wonderful dis- 

 coveries in Dialysis by Professor Graham, F.R.S., H.M. Master of the Mint, showed us 

 how crystalloids, such as carbonate of lime, could percolate through animal and vegetable 

 membranes. It is probably by the laws of Dialysis that we shall be enabled to find out 

 the process of the calcification of the specimens described in this Memoir. 



§ 4. General Remarks on Specimens of Calamites and Calamodendron. 



For a long time I have devoted considerable attention to the genus Calamites, and 

 have collected a tolerably good suite of specimens which show structure. There is no 

 difficulty in obtaining any quantity of fossil wood, showing the wedge-shaped bundles of 

 pseudo-vascular^ structure, which, springing in radiating series from certain circular and oval 

 orifices, next the central axis, are parted by wedge-shaped masses of very coarse cellular 

 tissue, increasing in the opposite direction to those first mentioned. It is well known 

 that the casts of the central axis, showing by their ribs and furrows the former position of 

 the wedge-shaped bundles, are found in most of our Coal-measures ; but when we look for 

 specimens affording outside characters of the woody cylinder, and examples of the pith or 

 central axis showing structure, we find great difficulty in obtaining them. Most collectors 

 select large specimens, and in these, although we may sometimes be so fortunate as to 

 meet with evidence of the outside of the plant better than a mere carbonaceous film, 

 generally we have not much chance of getting any indication of the pith or central axis. 

 The wedges of pseudo-vascular tissue, proceeding from their radiating orifices, are usually 

 found compressed close together, and the space formerly occupied by the pith or central 



' For a very excellent account of the petrification of wood, see Dr. Goeppert's ' Die Gattungen der 

 fossilen Pflanzen,' published at Bonn in 1841, a work of great research, and which does not appear to have 

 been much known in England, judging from the few references made to it. This learned author long 

 anticipated me in the discovery of the structure of the rootlets of Stiffmaria, although I was quite unaware 

 of it when I wrote my paper on the same subject published in the 'Quarterly Journal of the Geological 

 Society,' vol. xv (18.59), p. 79. 



^ The term "pseudo-vascular" is taken from Mr. Dawes' paper on Calamites published in vol. vii of 

 the 'Quarterly Journal of the Geological Society.' It may be a question whether "vascular" should not be 

 employed. 



