PALEOBOTANY BERRY. 293 



fine muds preserve every detail with great fidelity. Therefore, 

 shales, which are simply lithified muds, furnish beautiful fossils, as 

 in the roofing shales that overlie so many coal seams. The re- 

 deposition of calcium carbonate in the form of travertine often 

 entombs well-preserved plant remains as do the alluvial muds of 

 river flood plains. Volcanic dust falling in water forms an admir- 

 able matrix, as in the celebrated Miocene lake deposits at Florissant, 

 Colorado. Impressions of plants constitute the bulk of the objects 

 with which the paleobotanist has to deal. The substance of the in- 

 closed plant fragment may remain as a carbonaceous film; it may 

 be replaced by salts of iron or other mineral ; or it may be entirely 

 dissipated, leaving merely the impression. Occasionally lignified 

 remains, such as those from certain localities in the Upper Cre- 

 taceous, may retain their internal structure more or less intact, and 

 by special methods may be sectioned and studied microscopically. 

 The fire clays of the Coal Measures often furnish beautiful im- 

 pressions in dark tints on a light drab background, and some of the 

 light-colored clays of the Upper Cretaceous of South Carolina, 

 where the leaf substance has been replaced by iron oxide, yield hand- 

 some red impressions. 



Plant remains are often found in a good state of preservation in 

 nodules of iron or calcium carbonate. Coal or lignite beds are 

 simply examples of the inclusion of vegetable debris en masse. In 

 coarse sediments, or occasionally in finer materials such as tuffs or 

 travertine, more resistant objects like seeds or stem fragments have 

 left nothing but the cavity or cast, and such objects often furnish 

 satisfactory subjects for study. Amber or other fossil resin has 

 also frequently preserved mummies of plants or other organic re- 

 mains, especially delicate objects like flowers. The cuticularized 

 integument of pollen grains or spores is very resistant and is fre- 

 quently preserved. Similarly the cuticles of leaves retaining the 

 outlines of the epidermal cells and the stomata are often found to be 

 intact after the remaining tissues have become completely disorgan- 

 ized. These can sometimes be peeled from the fossils and mounted 

 for microscopical examination, or, if this is impossible, collodian 

 casts of the surface can be made and by the proper handling of the 

 illumination yield satisfactoiy microscopic details. 



The second and third methods of preservation, or more generally 

 a combination of infiltration and replacement, are usually termed 

 petrification. The internal structure of the plant material is con- 

 served with more or less perfection according to the rapidity of 

 permeation before the tissues have rotted and the completeness with 

 which replacement has taken place. If this happened before the 

 tissues became disorganized thin sections may be ground and the 

 136650°— 20 20 



