1897.] Fossils and Fossilization. 19 
face they were passing. The year was then one of sub-tropical 
mildness and warmth. Their excellent preservation in many 
instances, as well as the number and completeness of their 
skeletons, prove that their fossilization resulted from a sudden 
calamity and rapidly ensuing sepulture. It seems most likely 
that the floors of those ancient lakes were themselves abysses 
of mud, a soft calcareous and argillaceous silt into which the 
great mammals sank when dead, or if caught in overwhelm- 
ing floods were speedily enveloped in the accompanying tor- 
rents of earthy material. These remains have undergone a 
partial mineralization, and have been penetrated by the min- 
eral matrix quite extensively. In examining the typical col- 
lection of mammalian fossils from the White River of Dakota, 
from which Dr. Leidy made many of his species, I have ob- 
served the process of fossilization. The marrow cores of the 
leg-bones of Oreodon culbertsonti Leidy are almost closed with 
chalcedony quartz and calcite. The brain cavities of the 
same animal are filled with an exact mould of marl which in- 
dicates the pasty consistency of the original substance in which 
the skull was placed. The remarkable series of similar moulds 
used by Prof. Marsh (Monograph of Dinocerata) to illustrate 
the growth and specialization of the animal brain have been 
formed in the same way, retaining with fidelity the furrows 
and rugose character of the interior surfaces of the skull.? The 
interparietal spaces of the lower jaws of Hyracodon nebracensis 
Leidy are also invaded by clay and mineral matter, so as to 
partially mineralize the contiguous bone. In the leg-cores of 
the same animal a cement of argillaceous limestone with sep- 
arated grains of quartz and sometimes a solid stem of quartz 
filling the passages are noticeable. The bones of Menodus 
proutii Leidy are heavy from parietal petrifaction and replace- 
ment, and in some the cellular structure of the bone is satu- 
rated with chalcedony flakes and granules. It is quite cer- 
tain that the geological changes which have effected the ele- 
vation of these tertiary lakes and made them dry basins, 
? These brain moulds might not be strictly considered fossils, but they come 
within the application of our definition as an “indication of life” in the same 
way as casts of shells. 
