WHAT MAY BE LEARNED FROM EOZOON 137 



and are, become imbedded in clay, marl, or other soft sedi- 

 ment, they can be washed out and recovered in a condition 

 similar to that of recent specimens, except that their pores or 

 cells, if open, may be filled with the material of the matrix, or 

 if not so open that they can be thus filled, they may be more 

 or less incrusted with mineral deposits introduced by water 

 percolating the mass, or may even be completely filled up in 

 this way. But if such fossils are contained in hard rocks, they 

 usually fail, when these are broken, to show their external sur- 

 faces, and, breaking across with the containing rock, they ex- 

 hibit their internal structure merely, and this more or less 

 distinctly, according to the manner in which their cells or 

 cavities have been filled with mineral matter. Here the 

 microscope becomes of essential service, especially when the 

 structures are minute. A fragment of fossil wood which to 

 the naked eye is nothing but a dark stone, or a coral which is 

 merely a piece of grey or coloured marble, or a specimen of 

 common crystalline limestone made up originally of coral frag- 

 ments, presents, when sliced and magnified, the most perfect 

 and beautiful structure. In such cases it will be found that 

 ordinarily the original substance of the fossil remains in a more 

 or less altered state. Wood may be represented by dark lines 

 of coaly matter, or coral by its white or transparent calcareous 

 laminae; while the material which has been introduced, and 

 which fills the cavities, may so differ in colour, transparency, or 

 crystallization, as to act differently on light, and so reveal the 

 original structure. These fillings are very curious. Sometimes 

 they are mere earthy or muddy matter which has been washed 

 into the cavities. Sometimes they are transparent and crystal- 

 line. Often they are stained with oxide of iron or coaly 

 materials. They may consist of carbonate of lime, silica or 

 silicates, sulphate of baryta, oxides of iron, carbonate of iron, 

 iron pyrite, or sulphides of copper or lead, all of which are 

 common materials. They are sometimes so complicated that 



