192 PKOF. W. C. WILLIAMSON ON MINEKALIZATION. 



When yoa go to Egypt you find in the desert regions of the 

 Wadys Anseri and El Tih to the south of Cairo, fossilized 

 forests of trees, in some cases with stems 20 or 30 feet in 

 length, all converted into a form of Silica. Short stumps are 

 still standing as if they were growing in the sand. Thus we 

 have here examples where Silica does the same kind of work 

 that lime does elsewhere. 



There is always an obvious natural affinity between the 

 Silica and organic bodies ; the one has evidently a marked 

 facility for replacing the other. In the case of these fossil 

 plants in many instances the Silica has filled all the cavities, 

 but there is retained, enclosed in the Silica, the tissue that con- 

 stituted the cell walls and the corresponding walls of the 

 vessels of the plant. In other cases all these tissues have 

 disappeared — nothing is left but Silica, and yet there remain 

 sharply defined the position and extent and arrangement of 

 the carbonaceous tissues, just as perfectly as when the plant 

 was in a living state; one of the most remarkable of the 

 mineralizations of plants. 



When we turn to the animal kingdom, we discover some 

 exceedingly interesting cases of silicification. I have put 

 under the microscope two preparations from the Foramini- 

 ferous world. You are all aware that the soft animal 

 of a Foraminifer is a mere protoplasmic body — it has no vessels, 

 no skeleton — is nothing but a little speck of jelly-like matter. 

 Speaking of the animal as retained within its shell, a Rotalia 

 begins with a central cell, to which is added a second larger 

 chamber, but connecting it with the first one is a small per- 

 foration in the partition separating the two segments, and by 

 a succession of such growths is produced a spirally-arranged 

 group of segments, connected by a succession of minute necks. 



These chambers, when the animal was living and at rest, 

 were filled with this protoplasm, using a strict physiological 

 term instead of the now needless one of Sarcode, and we find 

 that the animal occupies all these chambers in the same way. 



Now, gentlemen, when you take some of these dried-up 

 Foraminifera that you so often get from foreign regions, you 

 apparently have only the shell, but you can sometimes, after 

 treating this shell with acid, get the dried animal separate 

 from the shell. I have on the table, obtained in that way, a 



