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sion on the contiguous fibre, whereby a small cavity is left between 

 them. These markings or cavities very rarely exist on the sides of 

 the fibres opposed to the pith or bark, but are very numerous on the 

 sides parallel to the medullary rays. Wherever the markings occur, 

 the saucer-shaped depression is thick at the circumference and for 

 some distance towards the centre ; but in the centre itself there is a 

 spot so extremely thin and minute, that the light, which has to pass 

 through it, becomes decomposed, and the spot looks either green or 

 red, according to the adjustment of the focus. 



"Having received from Professor Bailey a specimen of fossil wood 

 which was found at Fredericsberg in Virginia, I perceived, on sub- 

 mitting it to the microscope, that it would easily break into minute 

 fragments in the direction of the woody fibres, which, when carefully 

 viewed, presented a most beautiful example of casts of woody tissue, 

 with numerous spirals traversing the interior. At various points 

 were arranged the ordinary coniferous dots, and to the outside there 

 adhered small bodies of the same size, which projected beyond the 

 outline of the fibre when seen obliquely, each bearing the precise re- 

 presentation of the coniferous disk. In other parts of the field of 

 view were some of the same bodies detached from the sides of the 

 fibres, which left no doubt that they were casts of the cavities exist- 

 ing in the original plant, and proved the correctness of the view above 

 slated respecting the nature of these minute circular markings. Be- 

 sides these siliceous bodies in the fragments of the fossil, there were 

 others of such a shape as to leave no doubt that they were casts of 

 the interspaces between the cells or woody fibres. 



" There is very little doubt now, from the use of chemical tests, 

 that fossil woods for the most part, or perhaps in all cases, still pos- 

 sess portions of the vegetable tissues, which are cemented together 

 into a compact mass by silica, derived from the water to which the 

 specimen had been subjected. It is difficult to account for the lodge- 

 ment of silica in the tissues of plants ; but it is possible that the mo- 

 lecules of silica, which exist as one of their organic constituents, form 

 the first attractive points, to which others are added by the water, 

 until the whole of the portion of the plant, the woody fibres, the ves- 

 sels and cells, and the interspaces between these organs, is filled (in 

 fact all places which in the recent plant are filled with sap and air), 

 after the manner that the spicules of silica in a sponge form nuclei 

 for the subsequent deposits of flinty matter, until the whole is con- 

 verted into a shapeless mass like the original sponge. 



" It follows from these observations, as every fibre, cell and spiral 



