94 THE DAWN OF LIFE. 



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 intro- 

 duced by water, 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 surfaces, and, breaking across 

 with the containing rock, they exhibit their internal 

 structure merely, and this more or less distinctly, 

 according to the manner in which their cells or cavi- 

 ties have been filled. 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 gray or coloured marble, 

 or a specimen of common crystalline limestone made 

 up originally of coral fragments, 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 crystalline 

 structure, as to act differently on light, and so reveal 

 the structure. These fillings are very curious. Some- 

 times they are mere earthy or muddy matter. Some- 

 times they are pure and transparent and crystalline. 



