62 BOAKD OF AGRICULTURE. 



and depositing their fecal remains, and ultimately their bones 

 and teeth, — in fact, their dead bodies, — in these great open 

 crawls or pens." This explains how it happens that the re- 

 mains of these land animals are merely mechanically mingled 

 with and not imbedded in the phosphate nodules. 



But these fragments or nodules, broken off from the great 

 eocene marl-bed, rounded by the waves, and thrown by the 

 action of ocean storms into these great basins, were in the 

 form of carbonate of lime, like the mother-bed from which 

 they were detached. They remained almost pure carbonate 

 of lime, undoubtedly, long after the gradual and slow eleva- 

 tion or upheaval of the surface of the shallow coast. How 

 happens it, then, it may be asked, that they became phos- 

 phatic? What became of the carbonate of lime, and what 

 was the process of conversion ? 



Let us remember that these marl nodules, after the 

 upheaval or elevation of the surface, were lying age after age 

 in basius covered up with the accumulating fecal remains, 

 bones and carcases of land animals. In other words, they 

 were covered by deposits very rich in phosphoric acid. The 

 process of conversion of the carbonates into phosphates was 

 probably very analogous to that of petrifaction. We know 

 that if a piece of wood or other organic substance is long buried 

 or imbedded in clay, it is gradually changed into lignite, but 

 if it is buried in sand it will, under certain conditions, be 

 entirely silicified, or changed into silex or pure flintstone. 

 The organic matter will disappear, particle by particle, and 

 silica, in solution, will take its place. Heated water, if it 

 holds carbonic acid or an alkali in solution, will dissolve silica, 

 and this solution will percolate through the mass surrounding 

 the wood or other organic matter, and thus, atom by atom, 

 the silica is substituted in place of such organic matter, and 

 the exact form and outline, to the very innermost cell, will 

 be preserved, simply because the process is slow and takes 

 place only particle by particle. I have a fine specimen of 

 petrified wood from the desert at some distance out from 

 Cairo, in Egypt, where there is said to be a whole petrified 

 forest, in which the petrifaction is so perfect that the layers 

 of annual growth in the wood can be distinctly counted. 

 The whole is now a mass of pure silex. 



