io8o PRINCIPLES OF STRATIGRAPHY 



Petrifaction of non-mineral substances. 



(a) Replacement of soft animal tissue. As stated above, the 

 muscle tissues of a number of groups of vertebrates have been 

 known to be preserved in a most remarkable manner. In the upper 

 Devonic shales (Cleveland shales) of Ohio the muscular tissue 

 of cladodont sharks has been mineralized in such a perfect manner 

 that in places "they suggest in color, distinctness, and texture the 

 mummified tissue of recent fish." (Dean-io li"/.^.) Similarly, well- 

 preserved muscular tissue has been found in fishes of the litho- 

 graphic stone (Reis-20, pi. II), and in other deposits both finer 

 and coarser. The muscular mass thus preserved is pure mineral, 

 composed of about 80%-)- of calcium phosphate. Reis holds that the 

 muscular tissue was in a semi-decomposed condition, that minerali- 

 zation took place quickly, and that the remains must have been so 

 efifectively enclosed that decomposition was checked. The phos- 

 phate, he thinks, is derived from the body of the animal and precipi- 

 tated on contact of the decomposing material with the calcium car- 

 bonate of the surrounding sediment. Dean, on the other hand, 

 favors the view that the phosphate was deposited from solution 

 within the undecomposed tissue of the shark, which thus became 

 mineralized before it had time to decompose. The partial replace- 

 ment of human bodies mentioned above is analogous to the more 

 ancient case here cited. 



(b) Petrification of plants. ( Roth-22 :do5.) Aside from the 

 unicellular diatoms, in which the cell walls of the living plant are 

 impregnated with amorphous silica, and the unicellular to multi- 

 cellular lime-secreting algae, the tissues of plants may in general 

 be regarded as free from mineral matter. But plants immersed in 

 mineral waters, or buried where such waters have free access, are 

 saturated and completely impregnated with the mineral matter. 

 Colloidal silica is most favorable for the preservation of the delicate 

 cell structures, while calcite or other minerals of high crystallizing 

 power will cause deformation if not disruption and complete de- 

 struction of the cell walls. 



A mass of wood completely penetrated by and saturated with 

 silica still shows its original form and structure, even to the orna- 

 mentation of the cell walls, which, in a properly prepared slide, Avill 

 not appear very different from the fresh or dried tissues. Even 

 in appearance the impregnated -wood resembles the unaltered wood, 

 being fibrous and splintery, and the change is often noticeable only 

 from the difference in weight and hardness. The silica of wood 



