The PeII^CIPLES of PALAEO^^TOLOGY. 215 



in the case of fossil animals. The substance in question is usually 

 carbonate of lime ; sometimes silicate of magnesia, bisulphide of 

 iron, carbonate of copper, etc., or sometimes of argillaceous or 

 sandy particles. The fossil is found in a much higher degree of 

 preservation when the water holding in solution the mineral sub- 

 stances has been able to penetrate into the interior of the tissues. 

 The silica, the carbonate and phosphate of lime fill all the spaces 

 made by the cavities of the anatomical elements. Such petrifac- 

 tion takes place sometimes in plants remaining still in their place 

 of growth, sometimes in their remains which are transported 

 and accumulated in lacustrine waters strongly impregnated with 

 mineral substances. In such specimens the cellular parts and 

 their derivatives are sometimes preserved with all their orna- 

 mentation, and thinly-cut sections present precisely the same 

 details as do sections cut from living or dried tissues. If, on the 

 contrary, the fossils have been exposed to the air, decomposition 

 has more or less -completely destroyed the organic matter, and 

 there only remains a very porous mould of the C9.vities of the tis- 

 sue. In this case, before making sections, it is necessary to fill 

 the cavities with some fluid substance which will harden and 

 render the specimen compact. 



Plants are often preserved in considerable quantity without 

 the agency of mineral matter, in the state of lignite^ peat or coal. 

 These products are made by the incomplete decomposition of the 

 vegetable matter. The microscopic structure in this case is often 

 preserved in a remarkable manner ; in order to study this it is 

 necessary to make thin sections which are cleared by chemical 

 processes and studied in transparency under the microscope. 

 Details concerning the technicalities of this process, too compli- 

 cated to be explained here, may be found in the works of M. 

 Kenault and others. 



