[Chap. LII 



PLANTS OF THE PAST 



721 



Fig. 356. Side and end view of cast of a seed of Upper Carbonifeions age (A); 

 cast of a secjuoia cone from the Upper Cretaceous shales of Alaska. Photos b)' 

 W. Berry and G. C. Martin. 



mineral matter, the result is a cast (Fig. 356). If the substances of the 

 cell walls of plants are similarly replaced by mineral matter in suffi- 

 cient detail for one to recognize the cellular structure, the plant is said 

 to be petrified ( Fig. 357 ) . Some fossils are merely imprints ( Fig. 358 ) , 





yiro^ 



'\\'?>^'-^"#,^yi 





Fig. 357. Petrified trunks of trees that were living in this locality during the Triassic 

 period. Fossil Forest Park, Arizona. Photo by U. S. National Museum. 



